Ah yes, memories! I do have all that art, maybe I should dig it out and post it. I never drew the mutant whale kaiju though, it only exists in my head. I really enjoyed designing the kaiju and their alternate skin concepts, maybe I'll do some similar concepts in future, independent of the game.
Orangotango was actually designed to be more of a brawler that had weird Street Fighter/Dragon Ball-esque energy blast powers. His hair standing up and changing color was a reference to that. At one point I drew a sketch of him with a headband like Ryu. He never really caught on, though. And the green monkeys were created to go along with Gordon's mad scientist creator Dr. Ivo, who was all about mutations. I thought it would be funny if there were all these radioactive green mutant lab monkeys running around his labs shrieking and getting into trouble that he just sort've blithely ignored. They weren't a part of gameplay though, just a bit of visual fun added to the scientists.