just a manatee, the name is just probably a good on Wendell due to them being similar creatures. Although Manatees and Walruses aren't that closely related.
I assumed he was a manatee but maybe not. Now I wonder if maybe he is actually a dugong. dugongs have fluked tails which would work well as feet for a character.
edit also he is missing nails that manatees have, but that doesn't mean anything in cartoons.