I feel like I should point out that your topic implies you're looking for 20mm werewolves, rather than werewolves that fit on 20mm bases. I think your best bet for 20mm werewolves would be Thane's Wolfen.
(Actually, the more that I think about it, the more appealing attempting to conquer the Old World with an army of pygmy werewolves becomes.)
The Reaper werewolf apparently barely fits on a 25mm base. Maybe Blue Moon or (if you don't mind repurposing nazi werewolves (nazi werewolves are the worst)) West Wind?