I imagine the Ringwraiths, but the texture of the cloak is going to be smoother, less wrinkles.
Okay, here’s what Harry sees when he first meets a Dementor on the Hogwarts Express (From Prisoner of Azkaban, chapter Five, “The Dementor”:
“Standing in the doorway…was a cloaked figure that towered to the ceiling. Its face was completely hidden beneath its hood. Harry’s eyes darted downward…There was a hand protruding from the cloak and it was glistening, grayish, slimy-looking, and scabbed, like something dead that had decayed in water…the hand was suddenly withdrawn into the folds of its black cloak.”
So they’re definitely very tall, clothed in black robes. Sounds very much like a Ringwraith to me. But they’ll have a chance to make a definite distinction between the two later on in the movie when one of the dementors tries to administer the “kiss,” since that means we’ll see its face. If it’s anything like it is in my imagination, could be very, very creepy.