If this has been asked before just comment the question then I will hide it
He resembles a kind of tanuki or raccoon to me, but according to Bulbapedia, he is based off of a ferret.
Sentret is based off of a ferrett as its later evolution (Furrett) obviously looks like one. However Sentret to me looks like it's own unique Pokemon, possibly a Ferrett when its a baby? Whatever the cause may be, he is an amazing HM slave!
I personally think he looks like a rabbit. But he could be many other things.