Yeah, it's common where I'm from too. Along with all the infinite Max\maxwell,
Charlie's, Bellas, and baileys. I could go on and on
Riley could be used on any species, I wouldn't care much as long as if it was asigned to the male gender. And no, I do not view the name as unisex whatsoever. Grrr.
According to this: http://dogtime.com/top-100-dog-names.
Riley apparently is #13 among dogs. So yeah, it is quite a common name among them. However, still a very "humanized" name (: