The film makes it clear that Ape society is composed of all (non-Human) apes: chimpanzees, gorillas and orangutans. Back when the film was made, bonobos were thought to be chimpanzees, but nowadays they're known to be a separate species. This leads to some Fridge Horror: if bonobos are not present at all, it means they all died at some point (How? By whom?); on the other hand, if they're still around, given that we don't see them in the movies and the most notorious aspect of their society, it's not unreasonable to assume they are sex slaves, imprisoned in the homes of the other apes.
With bonobos, it's quite possible that if human knowledge on bonobos was incomplete back in those days, then all of ape society would be just as unaware of bonobos being a separate species. Therefore, it's not unreasonable to assume that bonobos were merely seen as chimps also and were assimilated into the chimpanzee sub-culture of ape society.
Also, the apes fit the stereotypes at the time: orangutans as leaders, gorillas as brutes, and chimpanzees as scientists. The first and last were inverted in the 2001 and 2011 reinventions given orangutans are solitary compared to the more social chimps - who were also given an uptick in aggressiveness, with gorillas in turn being more levelheaded (chimpanzees are prone to even attacking each other, while gorillas are usually calm unless disturbed).
Taylor's final rant at the end was thought up and written down by Charlton Heston on the day of shooting that scene. It adds much more power to that Wham Shot, even after repeat viewings.
The shot of the three judges doing the "See No Evil, Hear No Evil, Speak No Evil" pose wasn't meant to be in the movie, it was just the three actors goofing around between takes and they decided to put it in the film.
Zaius was originally going to be played by Edward G. Robinson, who got as far as filming the early test footage, but his ill health did not mix well with the heavy makeup, and he quit. The surviving footage shows Robinson playing Zaius as a cynical Deadpan Snarker, in contrast to Maurice Evans and his interpretation of Zaius as a bombastic authoritarian.
Also, Linda Harrison played Zira in early makeup test footage, but was later cast as the human Nova. (Both roles are due to dating the the studio head, but her former beauty pageant looks helped with the latter.)
Ursula Andress, Raquel Welch, and Angelique Pettyjohn were all offered the role of Nova, but either turned it down or were not available.
Ingrid Bergman was offered the role of Zira and turned it down. Later in life, she admitted to regretting her decision.
The early scripts for the movie had a closer resemblance to Pierre Boulle's novel, but budget constraints forced the change to the primitive ape society of the film.
Scenes were scripted and filmed revealing, near the end, that Nova was pregnant with Taylor's child. The scenes were cut out of the final print, as it was felt that they changed the focus of the ending, leaving the door open to a sequel Heston didn't want (but got anyway).
In the early scripts and the test footage, Taylor's name is John Thomas.
Interestingly, there was a sort of segregation behind the scenes during filming; the chimpanzee actors ate with other chimps, gorillas with gorillas, and orangutans with orangutans.
Once, some of the gorilla actors rode from the makeup area to the set in a convertible with the top down, wearing their full gorilla makeup, wigging out some observers on the way.