There's a lot of unusual animals I really love, but most of them are non-mammals, oops.
Since I don't wanna flood this thread with a lot of unusual species, here's one I recently found out about, the pea crab:

Look how small and cute! They're a species of small crab that are parasites of oysters.
They're about the size of a dime:

And yes they can even sometimes be found in the oysters you're eating, commonly cooked.