Sweet William

Sweet William is a large burly member of Cat R. Waul's Cactus Cat Gang.

An American Tail: Fievel Goes West
Williams just appears as a background character, like most of the other members of the gang

Fievel's American Tails
He plays bigger role in the show. In one episode, "Mail Order Mayhem", he plays out as the main bad guy, working with two other cats Slim and Felonius without Cat R. Waul's authority (as far as we know). In this episode he orders a "Mouse Slapper" which fires nets on mice, but was easily subdued by hired lawmouse Clint Mousewood. In another episode he reveals a soft side, taking a liking to Yasha Mousekewitz and finding her too adorable to eat when she wanders into his hideout in "Babysitting Blues".

It appears that after Waul was ousted from Green River at the end of Fievel Goes West, Sweet William no longer took orders from Waul unless there was something in it for him, and in fact beats Waul up in many episodes.