Grey Ones are first introduced in Three Against the Witch World.
Appearance: Grey Ones look like humanoid wolves. They can speak and in Wolfhead are noted to have tails and digitigrade feet. In many stories they don't seem to take care of their fur very well, and it is sometimes missing or in patches (likely due to mange).
Life Span: How long Grey Ones live is unknown. They could have been made humanoid from wolf stock, in which case they might have wolf-like life spans, though (as with the Keplian) they could have much extended lives thanks to the Power. They may have been humans transformed into something more wolf-like.
Power: Grey Ones are expert trackers and can thrice circle and bind. Following their trail directly leaves a person vulnerable to them. In The Warding of the Witch World a shaman seems to be able to call on some Power for illusion and they seem to be able to draw or track Alizonders, possibly due to the Alizonder canine affinity. Grey Ones could have additional abilities not mentioned in the books.
Other: Grey Ones hunt in packs like normal wolves. They are capable of both two-legged and four-legged locomotion. In Wolfhead it is suggested that the Grey Ones once used metal weapons and were created as a tie to animals. They are not, perhaps, wholly of the Light and so may have corrupted into use by the Dark like the Keplians were.