Ben as WildmuttEdit
Wildmutt appears to be a large orange dog with no eyes, ears, nose, or tail. His posture and movements are also somewhat apelike. His teeth are very defined and stick out of his mouth. Wildmutt has no eyes, instead using his sense of smell and hearing, which are aided by three gill-like nostrils located on each side of his neck.
Four year old Wildmutt looks very similar to his ten year old self, but a lot smaller and thinner.
10-year old Wildmutt in Ultimate Alien is almost identical to his 10-year old version in the original series, just with orange lips rather than black and the brace doesn't have any white in it. The Omnitrix symbol is also recolored.
11 year old Ben as Wildmutt in Omniverse looks almost the same as he does in the original series, with the exception of the fact that the Omnitrix symbol that is located on the brace that is on his shoulder is recolored to green.
Ben 10,000 as WildmuttEdit
At 30 years old, Wildmutt has grown larger. He has stripes on his back and has sprouted a tail. His appearance has shifted to a more feline form. Wildmutt wears the Omnitrix symbol on his forehead.