User blog comment:D2r/Unanswered Questions of Redwall/@comment-1298206-20100523034800

14: Uh..I dunno, maybe they migrated for some reason.

18: Yes, true. But in the first book, BJ was still starting to form the Redwall universe at that time. He probably decided to get rid of a bunch of stuff (Beaver, Horse, Pigs, Towns, Dog) afterwards.