I think it’s supposed to be a lemming. I imagine its name is just Lemmy, tho I call him Lem for short. Honestly though any small rodent is going to look like a rat or mouse to me (and I love animals lol.)
Looks like it is supposed to be a Lemming, as they refer to it on their github page (4th bullet comment). But I don't think they actually give it a name.
Also, fun fact, (and trigger warning) the Lemmy logo isn't symmetrical.