I guess we have to wait until someone figures out how to get access to the texture files and whatnot. In theory, it should be easy enough to rename texture files, but in practice... only if you know what they are and can find them.
There's some hidden variants of horses too - ones like the Black Snowflake Appaloosa, Grey Arabian, Sable Champagne MFT, etc. I haven't tested if any of them can also be stabled yet, but to my knowledge they don't appear anywhere in-game. While a couple of them look identical to other skins, the Black Snowflake and Grey Arabian both have their own textures (even if the Grey Arabian's look unfinished and weirdly pixelly). I'd guess though that if they can't be stabled, it'll be for a different reason than the gang horses.