Idling in Shattrath on my shiny new Albino Drake, I get a lot of people asking where it came from (the intended outcome!) as well as a lot of guildies racing to try to get one for themselves. I spent a lot of time pre-patch compiling a list of all the mounts that I had access to, and then set about buying, grinding, pvping, and otherwise working towards my 50 total. The list of mounts available in the game is quite long, and since not everyone can ride every mount, the numbers available to each person can vary by quite a lot. It can be quite confusing to work out which mounts you have access to, and which ones you're still missing.
So I thought I would share this information, in case people are still chasing their 50 mounts and need to know what their options are. For general info on mounts, visit the official WoW site - it will tell you where to buy your faction's mounts, for example, which I haven't outlined here.
Steps to collect 50 mounts for Leading the Cavalry
1. Buy your own faction's mounts (including the level 30 versions)
2. Buy mounts from the other races of your faction
3. Buy normal flying mounts and epic flying mounts
4. Grind Netherwing and Sha'tari Skyguard rep for drakes and rays
5. Grind Mag'har/Kurenai rep for Talbuks
6. Grind rep with Cenarion Expedition for the Hippogryph (1600g)
7. PvP - battlegrounds, Halaa
8. If you're still short - try some of the extras listed below.
Breakdown of the PvP mounts included in the amounts above:
So basically, unless you're a Tauren, you have the ability to buy, grind, or PvP your way to 50 mounts. You'll need to work towards them - through rep grinds, a lot of PvP, and earning some money (5000g for epic riding skill, the gold required for the vendor mounts, and the 1600g Hippogryph), but these are basically the "easiest" mounts to acquire without having to deal with RNG in instances and raids. NOTE: You can buy all 6 Netherwing Drakes; the first comes after you complete the final quest, but the other 5 can be purchased back on Netherwing Ledge from Drake Dealer Hurlunk.
If you still don't have enough, here are some extra options:
Once we hit Northrend there will be plenty more mounts injected into the game, and this guide is more to help people get their achievement straight away, without having to wait for the expansion. Here are some of the Northrend mount options.
Finally, only mounts that go into your mount tab count towards the achievements. Temporary mounts such as broomsticks do not count. Class summoned mounts (warlock/pally/DK) do count, as far as I am aware.