In the Planet of the Apes movie series, Aldo is the leader of the gorilla factions (and the ape revolution, by extension) during the rise of the ape society prior to humanity's downfall, as the "highest species" of the planet.
Aldo is first referenced in the third Apes movie, Escape from the Planet of the Apes, when the chimpanzee Cornelius describes him as the first ape to acquire the power of speech—and the first to say "No!" to his human captors. He appears in the next film, Conquest of the Planet of the Apes, (portrayed by actor David Chow) as a Chimpanzee whose beating at the hands of human guards is witnessed by Armando and Caesar, the son of Cornelius and Zira, who later leads the ape revolt.
In the fifth and final film, Battle for the Planet of the Apes, Aldo emerges (portrayed by actor Claude Akins) as the general of the gorilla militia, who mostly guard the outskirts of Ape City, and practice with mock weapons. He is both jealous of Caesar (and his son, Cornelius, who bests him in the school all apes must attend) and would like to take his place as leader, and of his human advisors. Humans live alongside apes in the city, but as second class citizens.
Discovering a large number of (irradiated) humans still live underground in the bomb-destroyed city they escaped, as he is discovered during a trip to the ruins, Caesar implores the apes to prepare themselves, in case those humans emerge to attack. Aldo uses the alarm as an excuse to corral all the humans of Ape City, break into the armory to seize guns and other weapons, and declare martial law, with himself as de facto leader. Meanwhile young Cornelius has been injured falling from a tree branch (purposely cut by Aldo), and Caesar doesn't want to leave his son's side, so Aldo can do as he pleases. Caesar only remembers his duty toward the other apes, when an attack indeed comes.
Caesar and Aldo fight together, but with the enemy driven back, Caesar discovers Aldo's usurpation of power, his treatment of the humans, and Aldo's role in his son's death. Caesar and Aldo battle it out, from the ground to the trees, and for ultimate control of the ape/human society.