“Furry” implies a degree of anthropomorphism that creates a clear divide in pony smut. You’ve got the anthro kind, which is furry porn, and you’ve got the feral kind, which is furry adjacent but not furry.
Dumb divide to make. Cause standing on two legs animals can be less anthropomorphized in behavior. It’s more of a gray line as opposed to a black and white one. Some animals would always be bipedal, does not mean they are ‘always’ furry to you, correct?
Furry is simply enjoying animals given human features, be them mental or physical. It doesn’t even need to be sexual. MLP ponies are furry due to be mentally human (feral as it’s called from my understanding of furries), so liking them do make you a furry.