In the Planet of the Apes movie series, Aldo is the leader of the Gorilla factions during the rise of the ape society prior to humanity's downfall, as the "lowest 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 film, Battle for the Planet of the Apes, Aldo emerges (portrayed by actor Claude Akins) as the general of the gorilla militia, which mostly guards the outskirts of Ape City, and practices with mock weapons. He is both jealous of his human advisors and of Caesar (and his son, Cornelius, who bests him in the school that all apes must attend) and would like to take his place as leader. Humans live alongside apes in the city, but as second class citizens.
Discovering a large number of (irradiated) humans still alive 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. Meanwhile, Cornelius has been injured falling from a tree branch (purposely cut by Aldo), and Caesar doesn't want to leave his son's side, therefore Aldo can do as he pleases. Caesar only remembers his duty toward the other apes when an attack comes.
Caesar and Aldo fight together, but with the enemy driven back, Aldo ignores Caesar's order to stand down, and leads his forces to catch and massacre the retreating mutant humans. Caesar discovers Aldo's usurpation of power, his treatment of the humans, and Aldo's role in his son's death. Upon Aldo's return from the slaughter, the two battle it out, from the ground to the trees, and for ultimate control of the ape/human society. According to ape lore, an ape had never killed another; the population, aghast at Aldo's sin, mournfully chant "ape has killed ape" while Aldo and Caesar fight in the tree. Caesar avenges Cornelius, knocking Aldo from the tree to his death.