honestly it really varies from person to person as of why they call themselves that. most i know, me included, just find comfort in dressing up like a dog (ears, tails, collars) and all that, just because it's silly and cute, for just wearing ears and tails for aesthetics it's called the kemonomimi. some people call themselves that due to pet regression (think like age regression), some due to nsfw stuff, sometimes terms "pup-" are used for a kink community, u might see those leather masks and all that (sometimes also worn just for aesthetics, not necessarily a kink). so it really depends!