It was screenwriter Paul Dehn who first developed the idea of a mutant population inhabiting a buried New York. He had been asked by producers Mort Abrahams and Arthur P. Jacobs to come up with a visually shocking sequel to their hit movie Planet of the Apes. While he could have been satisfied to include this race of mutants as one-dimensional monsters, Dehn went into tremendous detail to give the community a history and an origin, right from his earliest story outline. In that first treatment - titled Planet of the Apes Revisited and submitted on 13 September 1968 - he described a society including giants and dwarves, with extra digits and mis-shapen bodies. The deformities were not disguised as in the movie, but rather were (publicly at least) celebrated. Only two mutants were given names: Caspay, Taylor's dwarf inquisitor; and the 'Hereditary Leader of the Mutants', Mendez XXVI.
The nature of Mendez's leadership was given considerable attention. "Their hereditary leader, Mendez the twenty-sixth, never tires of telling them that this religion is the first in Earth's history to have been founded not on Faith but on Fact. And this, in a sense, is true. When the last Atom Bomb exploded in up-state New York (circ. 1995) and huge earth-subsidences buried New York City, the few thousand survivors on the surface went underground in the hope of avoiding or at least minimising the genetic effects radioactivity. It took only the misshapen birth of the next generation for this hope to be proved false. The mutants were 'created by the Holy Fall-Out descending like manna from the Divine Bomb'."
On his way to meet the Leader, Taylor remarks flippantly that he never thought he'd live to see New York under a monarchy, to which Caspay is unamused. Of all the mutants, Mendez seems the most 'un-human' in appearance. Inside Mendez's 'Throne Room', "Taylor confronts a figure who looks like the Grand Inquisitor grotesquely lengthened by a distorting-mirror. The immensely tall, cadaverously lean body is topped by a head discernibly human, though the great aquiline nose suggests an eagle's beak. Even the long fingers are raptorially curved like claws. But the eagle seems blind. Where there should be eyes, there is facial flesh. Then a curious thing happens. Mendez says: 'Let me look at you' and turns slowly into profile to reveal that his eyes are at the sides of his face and not in front. He can, like a great bird, look outwards but not forwards."
As Mendez is left alone again, we are given a glimpse into his history and the contradictions of his philosophy. "Mendez rising from his throne. A door slides back and he enters a long corridor, lined with statues, past which he unhurriedly stalks. At first we should think that the statues are semi-representational, impressionistic sculptures in the modern manner of our own 20th century - of, the small-headed woman of Henry Moore, the grotesque elongations of Giacometti, the half-human, half-bird-headed fantasies of Elizabeth Frank. But as we pan Mendez past them, it should rapidly dawn on us that they are strictly representational, totally realistic statues of Mutants; and the names and dates on the plinths tell us that they are Mendez's ancestors: Mendez XXV, Mendez XXIV, XXIII, XXII, XXI etc. As Mendez walks, we are so to speak moving back in time; and as the dates become earlier, each statue's appearance approximates closer to the human norm. The last few are no more grotesque than the busts of the in-bred, later Medici on the steps of the Uffizi in pre-war Florence: ears of unequal size; a tiny face lapped by folds of circumambient fat; an outsize nose, an undersized chin. We end on Mendez I - a normal, handsome, strong-jawed, military-moustached, grizzle-haired soldier in the 20th century uniform of a five-star US General. The date: 1997 - B.3. In front of this, Mendez pauses for a moment - seeing (without turning) out of his side-eye - and whispers in a sort of agony: 'Forefather, why can we not all look like you?' This shocks us, because it is directly contrary to his public teaching that all Mutants are beautiful." The implication, never explicitly stated in any of Dehn's works, is that the House of Mendez is a hereditary monarchy stretching back to the fall of mankind in the 1990s, and that there have been twenty six leaders of the community, all named Mendez. Despite the fact that they would have to each have had exceptionally long reigns - perhaps explained by their mutations - there is no suggestion that there were monarchs with other names alternating with them, as was the case in the historical monarchies and the Papacy that this dynasty was inspired by. Dehn also points out, even at this early stage of development, that the founder of the Dynasty was a military General - an idea he would remain faithful to when revisiting the concept four years later.
At the treatment's conclusion, as the apes launch an invasion of the Mutant City, Mendez again ponders his fate and his cursed inheritance, asking his dynasty's founder "Forefather, what will happen to us, if we survive this day? From whom will come the new, untainted blood that should cleanse our inbred race? We have lived a thousand years alone, like worms, below ground - doing good to none but ourselves. If we come up to Earth's surface we shall die. And though my people have been taught (for their greater comfort) that they are beautiful and therefore to be desired, what stranger, coming down among us, would find us desirable enough to use our deviant bodies for the creation of a nobler breed? Forefather, why can we not all look like you?" The deformities and mutations apparent in Mendez are due to inbreeding as well as to radiation, and Mendez is under no illusion that he is perfect, as the other mutants proclaim themselves. Indeed, this treatment draws clear parallels between the ape and mutant societies. Only the mutant government know about the existence of the apes and primitive humans; it is kept a secret from the wider community just as the human heritage is a secret guarded by the orangutans. When Taylor told Mendez of Dr. Zaius' maxim that "Even an enlightened society should be kept ignorant of any knowledge that threatens its existence", he expected Mendez to be contemptuous; instead Mendez was "deeply interested" in this philosophy. Caspay witnesses the gorilla army's altering of the Alpha-Omega Bomb so that it aims at the underground city, and he desperately tries to stop Mendez from firing it and destroying their culture. Mendez, in a locked room, may not hear Caspay but given his private musings, it's probable that he knows exactly what the consequences will be as he pushes the button.
As Dehn got the green light to write a screenplay based on his treatment, a few alterations were deemed necessary: "At first, we were going to have them really mutated with monstrous noses and three eyes, real horror figures, but we didn't think that would have been nice for the children and after a great deal of research, it was the makeup department that came up with the idea that if you had been radiated, all seven layers of your skin would have been destroyed, and all that would be left was this terrible network of veins." Director Ted Post was apparently the person responsible for the final look for the mutants. He remembered a drawing from a medical text entitled 'Gray's Anatomy', in which was printed a vivid picture of a man's head, with the top layer of epidermis removed. He suggested the idea to Dan Striepeke and John Chambers. With the magic of their skills, they transformed this into film reality. In order to create the effect of a human being mutated through exposure to radiation, make-up artist Chambers examined photographs of corpses where the outer dermal layer of skin had been removed. After sifting through other, more graphic potential mutant designs, he decided upon using these photographs for the basis of his mutant make-up. 
The change in the mutants' look led to necessary changes for Dehn's first draft screenplay (20 December 1968), as did the introduction of a new central character - Brent. In this version, the seemingly human Mendez is a less troubled character, and the Dynasty's history is observed through the eyes of the captive astronaut. "Guards lead Brent down a long white corridor lined on one side by head-and-shoulder portraits - each titled and dated. We pan them slowly from Brent's P.O.V., for they visually encapsule an aspect of the City's history. We start on an antique, cracked canvas in an old-fashioned gilded frame, labelled: Mendez I: 1997-B.3. The portrait is that of a handsome, strong-jawed, dark-haired, military-moustached soldier in the 20th century uniform of a U.S. Army five-star General. His likeness survives in the portrait of Mendez II - V, though the uniform has been replaced by clothes and (later) robes of an advancing fashion. The style of painting and framing advances, too; for the portraits are all by different hands. As we pan past Mendez VI - XIII, a strain of no more than ordinary depravity appears to be developing in the Mendez dynasty, as it once developed in the Hanovarians and the Medici: coarser features, a fatter face, a weaker chin, a balder head; possible evidence of alcoholism in a bulbous nose; and of brawling, in an eye-patch. But Mendez XIII, a handsome enough rogue, must have made a good marriage; because Mendez XIV (who appears to have succeeded him when young) is of a beauty sufficiently remarkable to halt Brent in his stride. But the giant guards will brook no delay and threateningly hurry him past the remaining portraits, which are all handsome. And the new, healthier infusion into the dynastic strain survives to the last portrait of all: the haughty, noble, austerely aqualine face (with the characteristically deep-set eyes and delicately-defined lip line) of Mendez XXVI. We have reached the corridor's end. The guards usher Brent down the steps of a narrow passage... the portrait's (enthroned) original is looking down at Brent below." Thus, Dehn cleaverly keeps his gallery of leaders, but uses it to show their deformities suddenly stop mid-way through their history. Of course, the reason for the sudden youthful improvement in looks is not attributable to genetics as Brent supposes, but rather to the adoption of facial masks to conceal the increasing mutations. Mendez XIV, it seems, introduced this tradition. The script also suggests that there is a biological succession ("Mendez XIII... must have made a good marriage"), but this is presumably also from Brent's flawed assumptions, and so cannot be taken as established fact.
By the time the final screenplay for the now-renamed Beneath the Planet of the Apes was issued on 10 March 1969, Dehn had down-played his Mendez gallery drastically. "Guards lead Brent down a corridor lined on one side by marble busts (of the Mendez Dynasty) which we need not identify until the corridor's end. Here the guards usher Brent down the steps of a narrow passage... and track in to the last bust. A plaque at its base proclaims: Mendez XXVI." Dehn's ingenious device for visually demonstrating the evolution of the mutants was abandoned, and while the Corridor of Statues was still used (reverting to the statues of the original treatment rather than a portrait gallery), the unidentified busts were all identical, with only colour and plinth varying. They were smashed by the invading ape army, following the example of Dr. Zaius who viewed them as blasphemous, in a scene filled with symbolism.
Paul Dehn continued to script further Apes sequels, adding to, and building towards, the story he had written for Beneath. His earliest draft outline for the fifth and final movie, dated 5 July 1972 and titled The Battle for the Planet of the Apes, allowed him the opportunity to show the character he had described in his very first Apes treatment - Mendez I. This outline was clearly intended to bring the saga full-circle and set up the events of Beneath: 2004 - On the thirteenth anniversary of 'The Night of Fires', 90% of mankind is under ape control, with only pockets of human resistance in the North. Nimrod ("a grizzled giant by John Wayne out of Royal Dano" according to the outline), dressed in the battered uniform of a US Army General, leads this resistance and his forces are heading south in their makeshift convoy to finish the war against the apes. The ruthless Nimrod destroys Caesar's 'Modern City' in an atomic holocaust, and later tells a mutated human survivor that his real name is General Mendez (a footnote says "we met Mendez XXVII (sic) in Apes II").
John William Corrington & Joyce Hooper Corrington wrote a new story outline in September 1972 (Epic of the Planet of the Apes), loosely inspired by that of Dehn, which relegated Mendez to a much more subordinate role. Here, Mendez was an electronics expert and Governor Breck's Communications Officer. The last-minute survival of Breck at the conclusion of the preceding film in the series, Conquest of the Planet of the Apes, made him a much more obvious and familiar choice to lead the human resistance to Caesar. Thus Mendez was included as only a minor character purely to tie-in with his descendent from Beneath. As it turned out, the actor who played Breck did not want to reprise the role. Instead, in the Corringtons' revised screenplay from December 1972, Breck's place was taken by by one of his assistants from Conquest - Kolp. This final version - Battle for the Planet of the Apes - described Mendez as Kolp's first lieutenant "who will one day be the first in the mutant dynasty that ends with Mendez XXVI in 'Beneath'" (Alma was now Kolp's Communications Officer). These alterations robbed Dehn of his chance to bring the Mendez Dynasty full-circle in the way he had envisioned all along. Ultimately, even the character written by the Corringtons was edited out of the theatrical release, so that the fate of the planet could be left to the interpretation of the audience members as to whether the race of mutants and the destruction of the Earth shown in Beneath were inevitable. The scenes with Mendez have since been restored to some DVD releases of Battle, with the result that the 'Corrington Mendez' is now generally considered the first of the Mendez Dynasty.
Drew Gaska's 2018 novel Death of the Planet of the Apes gave added significance to the effigies of the Mendez dynasty. Each one held the preserved memories of a former leader, and when the leader of the mutants 'communed' with his ancestors he was quite literally accessing their memories for precedents and guidance. Through the bust of Mendez I, Taylor was able to witness the nuclear war that founded the community: Grand Central Terminal devastated by a nuclear holocaust; missiles triggering a fault line at New York City; the area around Grand Central Terminal buried under radioactive rubble; a military train arriving with a deadly cargo; General Mendez placing the area under martial law and conscripting the civilians; decades later, General Mendez deciding to keep secret the existence of other survivors on the planet's surface. Meanwhile, the bust of Mendez XIV reveals the first encounter with the 'Inheritors' - who had defeated the 'Makers' - during his reign.
- Planet of the Apes Revisited - A treatment at Hunter's Planet of the Apes Scripts Archive
- 'Cinefantastique Planet of the Apes Issue' (1972)
- Behind the Planet of the Apes; 20th Century Fox, 1998
- Planet of the Apes Revisited - First Draft Screenplay at Hunter's Planet of the Apes Scripts Archive
- Beneath the Planet of the Apes - Final Screenplay at Hunter's Planet of the Apes Scripts Archive
- The Battle for the Planet of the Apes - First Draft Story Outline at Hunter's Planet of the Apes Scripts Archive
- Epic of the Planet of the Apes - First Draft Story Outline at Hunter's Planet of the Apes Scripts Archive
- Battle for the Planet of the Apes - Revised Screenplay at Hunter's Planet of the Apes Scripts Archive