Bingo! Optimus Prime as a gorilla... I can't do it.
This is a common misconception and a good example of why it's worth giving a show a chance before judging it. Optimus Primal is NOT Optimus Prime. If the fact that he has a different name doesn't clue you in, then the fact that he actually MEETS Optimus Prime in the second and third seasons should.
I've also never understood why people have a problem with the concept of Transformers, whose very nature is to adapt to their surrounding environments, adapting to animal forms if that is the dominant life form around them. IMO this took the concept to a new level that had only been previously touched upon by G1 lightly with Headmasters/Pretenders etc. in terms of biomechanical tech.