In the original draft, the horses the apes rode, also spoke. However the costumes were too difficult to pull off with the mouths lining up, so they decided to only use only talking apes. I bet you didn't know that
I believe that's a common misconception. The original script actually had horses riding upon apes. However, in screen tests the two men required to power the horse costumes kept falling apart a top those in ape costumes.
Thus the whole Planet Of The Horses concept had to be abandoned and they went with the simpler one man costumes required for Planet Of The Apes...