Crabs are fun little guys and I want to see them walking around sideways. Increased reach would be helpful because the default is really short for the kind of game it is. The reach would be the easiest to change via modding, though. Assuming every sane Minecraft player plays with mods these days, crab would be least helpful.

Armadillo, I dunno, I don't really know what an armadillo is or what it does in real life. Pet wolf safety has way bigger problems than what armour would solve. They walk off cliffs or get stuck in water all the time.

Penguins also sound fun for the environment, as long as their behaviour includes running and then skating on their bellies. Very important aspect. Being amphibious would also be interesting to see. I don't care about the boat speed thing.

I think crab is my favourite. I bet we'd get some awesome meme gifs out of it too, since crab is already a meme, Minecraft crab has such good potential.