I wasn't around for BattlegroundForums, don't want to steal any valorthere might've been multiple instances of different bugs that all resulted into the same
(also you forgot about battlegroundforums in your signature but yeah)
The bug is fairly new, on the day of ICC launch (same patch they introduced the Collections page in WotLK Classic), if you created a level one character it would have the riding skill, and it could mount pre-20. It was patched very quickly, I tried making a character on that day and it had already been fixed. There is a small number of players who have it grandfathered, and an even smaller number of players who plan to twink on them. Ultimately will be irrelevant, as SoD is coming in two weeks and everyone will be there, and not Wrath.https://xpoff.com/threads/did-any-39-twinks-manage-to-get-epic-riding.96725/
Probably from this back in TBC prepatch, or there's a new bug and I'm sad I missed it.