Fluffy the Rancor

The scary thing is, this is actually feasible. Rancors are actually sentient if you go by the Courtship of Princess Leia novel. The rancor that Luke fights in the movie was just a baby rancor; it was driven insane to get it to fight in the arena.

A lightsaber big enough for an adult rancor to use would probably be able to slice through one of those giant Imperial walker's legs O.o