Nice game! I like how the palette changes when you switch between animals. I like the art for each of the characters. You especially did really good with the title screen and the cover image art on the submission page. The cover art especially really catches your eye.
I like the mechanic of switching animals for different situations and that there were different paths you could take. My favorite is definitely the monkey as leaping along walls is a lot of fun. My least favorite is the frog. He tends to slide off the edge of platforms quite a bit which made for some frustration and the controls for the frog were really awkward.
Overall, this is a really interesting idea for a game. I could see this being expanded on with different levels making use of different animals' abilities or maybe even more transformations. Nice job!