You mean something like devolving? Humans who revert to earlier animals? I haven't seen a "mammals only" rule among the very few furries I have seen online. I have a feeling that the definition of furry is equally fuzzy and a bit of a hairy question to tackle (punk intended
).

This isn't any less a furry than things with fur are.

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Sorry, saw this earlier, but life got all hectic and I wasn't able to get back to it at that time.
Devolving? Depends on what you mean by that. If you mean Brantley reverts to an earlier form of human ancestor or ancestral animal, no. He's the surgically-produced equivalent of a human/Old One (in the Lovecraftian sense), ore more precisely, a human/Cthuloid Martian hybrid with the dimension/time travelling abilities of a being such as Chthulhu, Cthuga or Hastur - complete with tentacles and all, albeit still roughly human-shaped and -sized (in a later RP than the one in which he originated (
The Log of the HMAS Badger), he retained enough humanity, including human proportions, after transforming to be able to draw and fire his mauser and kill a priestess of a renegade reptilian order of Old Ones Worshippers who fell afoul of the very beings they claimed to worship (I did say "antihero," LOL).
The original intent of his surgical transmutation (as originally delineated in the RP
Extended Steam Salon), was to artificially create a "First Ancestor" of a proposed Timelord race to rival and stand against the Gallifreyans (yeah, we borrowed pretty heavily from various fandoms in that one, LOL. I get kind of ridiculous when let myself have totally free rein...). What actually happened was the creation of a human/cthulid monster that, Frankensteinian style, rebelled against it's creators and went on a centuries-long quest to bring down the Cthuloid Martian empire - which is more or less what happened, except that it was mainly by the efforts of his companions and the Pan-Dimensional regulatory organization that he joined up with, than by Brantley himself.
There's a lot more to his lore (lol) than that, but that's basically what he is and where he came from.
I had a pic of him that I drew years ago for an avatar on a Furry Fandom website, but Danged if I can find it now. Oh well, maybe I can whip one out sometime later...