'Old' Zao was an elderly retired orangutan of Ape City. He had been Minister of Science for thirty years, succeeding Dr. Augustus and preceding Augustus' son Dr. Zaius. He spent his retirement playing board games, sharing gossip and trading insults with his old rival Sabian, who had also served on the High Council. Sabian had Zao arrested when he challenged the code of silence surrounding Colonel Taylor, who had interrupted one of their games during his escape attempt. Zao believed that a tribe of talking humans must exist somewhere.
Held in custody for days by Marcus, both Zao and Mungwortt were secretly transported to the Forbidden Zone by Marcus' successor Cerek, acting on the instructions of Zaius. There, they were thrown into a pit at night and soon found themselves surrounded by the 'White Ones' - seemingly some sort of mutated or hybrid ape/human race of carnivores who had hairless ape-shaped bodies and white hair and fur on their flat-nosed faces and heads. This was the standard practice for the most dangerous ape dissidents, thus avoiding the sacred law "Ape Shall Not Kill Ape". After a struggle with the creatures, Zao was mortally wounded. After his death his corpse was devoured by the White Ones, leaving just his bones. Mungwortt continued his journey with the skull of Zao, who he imagined to be guiding and advising him long after Zao's death.
- Zao is exclusive to the 2011 novel Conspiracy of the Planet of the Apes and it's 2018 sequel Death of the Planet of the Apes.