WoW Insider writes:
"Yes, that's right. You better start looking at ranch property because you have 100 mounts to collect. Here's the series of Achievements involved:
1. Stable Keeper
2. Filling up the Barn
3. Leading the Calvary
4. Mountain of Mounts (Alliance/Horde)
And what is your reward? Why, more mounts, of course! The 50 Mount Achievement will net you an Albino Drake (and put you mount closer to the final achievement.) The 100 Mount Achievement is separated into Horde and Alliance rewards due to racial mounts making for different requirements. The Horde 100 reward is Red Dragonhawk Mount and the Alliance 100 reward is the Blue Dragonhawk Mount."