They were both used in Classic. The brown one is just another texture that was available and was used for certain skele nameds. I believe it was a necro pet at one point too. Screenshots aren't that difficult to find. There is one
here. And
here.
It is the same exact race ID as the white one just a texture. It is the client that is stopping it from showing up normally because it has some weird hardcoded nonsense in it but pretty much everyone just has it sitting in their files.