Currently we are loading patch 1.5.x and the "old school ride" (wow achievement name) mounts have been removed in a recent update. This is not following the vanilla timeline. Every database and patch records shows the mount changes in patch 1.12.1.
Here are 2 such records:
Wowwiki
http://wowwiki.wikia.com/wiki/Patches/1.x
Wowhead (have to read when the item was introduced into the game)
http://www.wowhead.com/item=18776/swift-palomino
I do not know why they did this so far early, maybe to make them more rare. I personally think it would be better if they removed them after Naxx has been cleared (or released if it takes a long time) with more accordance with the actual vanilla timeline. It would be a token to show who played during the entirety of progression vs MC & Onyxia's Lair.