Board Thread:General Discussion/@comment-28446618-20160611074631/@comment-27441478-20170108203255

I think it was pretty obvious both in-universe and officially by the publishers that he was established a hedgehog, whilst chip has not been exactly assigned to a specific species.

And I think sonly has only been called to as a 'rat' by only Doc Robotnik too and no one else, whilst chip has been referred to as a Chihuahua by more than one person.

Just a theory.