In terms of other sentient species, I know of a few, though they're not really that common unless you go to their own territory so it's mostly just humans all the way down.

In terms of non-human contact, I've spoken with vampires, werewolves, ghosts, a few fairies, the occasional half-divinity, elementals of various types, far too many magic users with delusions of grandeur, and bees.

[Despite how definitively Daniel gave this list, (counting them off on his fingers no less) he is leaving something out. The young man leans back in his chair, directing a questioning glace at Celestia.] Your turn, outside of ponies, what sentient species do you have back home?