At first I thought you meant like, an actual dog (like the previous Sebastian, I know there's a theory they're in fact the same), but now I'm thinking maybe like Ciel is the Queen's "watch dog" ? :o sorry if I'm misunderstanding.

Like he already knew the family ? Honestly it's an interesting theory, after all Vincent knew UnderTaker already, and I wonder how much he knew about him and that he's not human. So why not ? Maybe he himself dealt in some shady black magic stuff. Or it was just a coincidence that Sebastian happened to be in a contract when he met the Phantomhives, and saw the twins when they were really young. I do love the trope of "we actually met before we (officially) met" so that would be funny to explore  !