Kilgore Trout is the pseudonym for Philip Jose Farmer. Farmer was a big fan of Vonnegut. During a writing dry spell Farmer asked Vonnegut for permission to use the Kilgore Trout name. Vonnegut is said to believe that Trout has gotten out of hand and will not allow the Trout name to used any longer.

Interesting Links:
Vonnegut comment
Farmer response