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A Horde of Hoofbeats
Obtain 400 mounts (usable by a single character).
A Horde of Hoofbeats rewards the 
Frenzied Feltalon mount for obtaining 400 mounts usable by a single character.
Understand the Mount Counter
The mount counter for this achievement does not update in real time with every mount learned — it can batch-update after multiple mounts are collected, so do not be alarmed if the counter does not move immediately after learning a single mount.
Use Class Mounts to Boost Count
Characters on classes such as Paladin, Death Knight, and Warlock have class-specific mounts that add to your counter when logged in on those characters; once the counter updates to a higher number, it remains at that number even after logging back to a character without class mounts, so logging onto a class with class mounts is a reliable way to push your counter higher.
Check the Preceding Achievement
The 
Biting Frostshard Core is awarded at 350 mounts from the preceding achievement No Stable Big Enough, so reaching that milestone first is a natural checkpoint on the way to 400.
Pursue Remaining Vendor Mounts
If you stall near 400 mounts, faction vendor mounts and Auction House mounts are available as a final push to reach the threshold, noting that some are priced very high.
Tips
There are currently 497 mounts that can count toward this achievement's counter, giving substantial room beyond the 400 required.
Logging onto a class character with class mounts before checking your counter ensures the highest possible count is reflected, which can reveal you are closer to 400 than your main character's counter suggests.