Leading the Cavalry

Leading the Cavalry is a collection achievement earned for learning how to summon 50 different mounts on a single character. Temporary items that let you ride a mount do not count towards this achievement. Upon completion, Mei Francis will mail you the Reins of the Albino Drake, a swift flying mount. With the pandaren mounts added to the factional mounts, the total normal mount count is up to 44, making this achievement much easier.

Strategy

 * At level 40, when you have exalted reputation with all your faction's home cities, you can acquire 44 mounts from these cities. (Alliance: don't neglect Black Stallion Bridle.)
 * At level 70, you can buy 7 gryphons or wyverns in Shadowmoon Valley, Hellfire Peninsula, Borean Tundra or Dalaran. (In Cataclysm you can now buy them near the Orgrimmar and Stormwind flight masters.)
 * You can acquire 1 mount from the Stormpike Guard or Frostwolf clan, and 5 more mounts from the Hall of Legends or Champions' Hall, for Honor points.
 * When you are exalted with the Kurenai (Alliance) or Mag'har (Horde) faction, you can purchase 4 Riding Talbuks and 4 War Talbuks.
 * When you are exalted with the Sha'tari Skyguard you can get 5 nether ray mounts.
 * When you are exalted with the Cenarion Expedition you may purchase the Cenarion War Hippogryph for 1,600 gold.
 * When you are exalted with the Netherwing faction, you can get 6 Netherwing drake mounts.
 * When you are exalted with the Wyrmrest Accord you may purchase the Reins of the Red Drake for 1,600 gold.
 * in Dalaran sells 4 different mounts.
 * Completing The Culling of Time, a relatively easy achievement, drops Reins of the Bronze Drake.
 * Grand Master tailors can build 3 flying mounts that only tailors of appropriate skill level can use.
 * Grand Master engineers can build 2 flying mounts that only engineers of appropriate skill level can use.
 * Grand Master engineers can build one ground mount that can be used by anyone, but it is tremendously expensive - money that would be better spent on buying mounts from vendors first.
 * Warlocks and paladins get two summonable mounts at levels 20 and 40. Death knights get a summonable mount at level 55.
 * Alliance characters who are exalted with The Winterspring Trainers can also purchase the Reins of the Winterspring Frostsaber. Horde characters can do the same in Un'Goro Crater for the Whistle of the Venomhide Ravasaur.
 * The Temple of Ahn'Qiraj has random drops of four mounts (red, green, blue, and yellow) that can only be used within that raid instance, however they still count towards the achievement.

Thinking ahead
If you are starting a new character with the intention of getting this achievement and, then there are a few ways you can plan ahead, some of which are listed above.
 * Choose a Warlock or a Death Knight for two free mounts or a Paladin for three mounts.
 * Become an Engineer or a Tailor. You can train these professions high enough to make these mounts and then drop the profession, if you chose to - you will no longer be able to use the mount, but it will still be in your mounts and count towards the achievement. You can be both professions at the same time, but it is very expensive if you don't already have an alt character with mining.
 * If you choose a Druid, know that flight form and epic flight form do not count towards the total number of mounts.