Pluto, also known as the Demon Dog, is a large dog that terrorizes the town of Houndsworth until Sebastian tames him. Afterward, he lives at the Phantomhive Estate. He can breathe fire, walk through barriers and shape-shift into a human. As a human, he retains dog-like qualities. Although Sebastian hates dogs, Pluto has taken a liking to him and gets jealous when he receives less attention than a cat. He only listens to Sebastian's orders. Finnian is in charge of walking him, as he is the only one strong enough to do so.

Pluto appears only in the anime adaptation

(Source: Wikipedia)