RS was weird about them being animals.
The first season had a bunch of anthro background characters, but then they pretty much just had humans. Anthros only showed up in bit roles from then on.
Pops named Rigby a little raccoon ones, but generally, the staff preferred them as just dudes. Every animal trait, like Rugby eating from the garbage dump or them being naked was just a gag. BUt I thought it was always at odds with the more serious element of the show.