While in the magical world of computer gaming nothing is impossible, there are many things, like ridable gorillas, that are just down right unlikely.
One of the main reasons we haven't done it is that gorillas chaff easily when being ridden by a player wearing armor. When that happens, you can believe the Britannian Brotherhood of Beastie Betterment will get cranky. I've seen the BBBB angry, you wouldn't like them when they're angry.
On the technical side, the problem is the animations required. You will notice that all current mounts allow us to use the same animations for the player, we just need to adjust the character so their bottom is on the back of the creature... to ride on an ape would be a very different kind of animation. Furthermore, we would need to make animations for attacking/casting spells, etc while in this unique position. Finally, we would need to make special animations for all the clothes since they are actually animated separately. I think you can see why there will probably be no ridable gorillas for UO.
That said, I hope you enjoy the number of ridables we already have, which is a lot. And I can assure you that we will keep making new ridables.