In Transformers: Rise of the Beasts, the Maximal who transforms into a gorilla is Optimus Primal.
Optimus Primal is the leader of the Maximals, a group of robotic beings who can transform into animals. In the film, his alternate form is a western lowland gorilla, making him visually distinct from the vehicle-based Autobots and Terrorcons.
The character originated in the Beast Wars era of the Transformers franchise, where Optimus Primal was the Maximal leader and transformed into a gorilla. His name deliberately echoes Optimus Prime, the Autobot leader, although the two are separate characters.
Rhinox, Cheetor and Airazor are also Maximals in the film, but they transform into a rhinoceros, cheetah and peregrine falcon respectively. Optimus Primal should therefore not be confused with Optimus Prime, who transforms into a truck.