WOW 100 mounts achievements guide

Stable Keeper
Filling up the Barn
Leading the Calvary
Mountain of Mounts (Alliance/Horde)
And what is your reward? Why, more mounts, of course! The 50 WOW Mount Achievements 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.

I was going to compile a list of all the mounts available in the game and then categorize them by group for you to pick and choose which mounts of the 100+ available were the best for you to pursue. But a little Googling showed me that this has already been done. So here are some excellent lists for you to read to plan which mounts you are going to go after:

World of Raids Mount List
WoWVaults Mount List
And for tracking your current mounts, I can't more highly recommend Bennlinger's WoW Mount Check List.

The lists generally break down like this:

Faction mounts
There's your racial mounts (remember, you can buy more than one) and the mounts of the other races in your faction. Take advantage of the Lunar Festival to help you grind to Exalted with the other racial factions, as each Elder you talk to gives you +75 faction across all of them. Bonus, as of WOW Patch 3.0.8 you can actually ride the mounts of other races as well. You can also do the quests in the starter zones of each race to gain faction with that area's capital as well.

Then there are the Burning Crusade faction mounts:

Shadowmoon Valley flying mounts
Kurenai/Mag'har
Netherwing
Cenarion Expedition
Sha'tari Skyguard
And the Wrath faction mounts:

Wyrmrest Accord
Sons of Hodir
Kirin Tor (no faction necesarry for these mounts, just gold)
There are a total of 80 mounts for Horde players and 84 for Alliance players. This is where the bulk of your mount collection will be coming from.

PvP Mounts

There are 5 mounts from Alterac Valley, Warsong Gulch and Arathi Basin tokens, 2 from AV tokens, 2 from Nagrand's Halaa tokens and 1 from Wintergrasp. You can probably snag these if you are an avid PvP'er as they are still available. The flying mounts from winning each season are retired.

Crafted Mounts

Mostly restricted to the crafters that make them. Tailors have two versions of their flying carpets. Engineers have their two flying machines and their chopper that non-engineers can ride (Mechano-hog/Mekgineer's Chopper). You can buy one off the AH or commission one from an Engineer. Either way, you're looking at 16k gold.

TCG Mounts

There are five of these, but you are looking at laying out actual cash to get your hands on them. Fortunately, you can avoid them entirely.

WOW Achievement Mounts
The proto-drakes are very involved to acquire, but I'm listing them here because, hey, it's a column about Achievements, right?

The Albino Drake mount for the 50 WOW Mounts Achievement counts as your 51st.
The Black War Bear is the reward for the Achievement killing the opposite faction's leaders (For the Horde/For the Alliance). Now is a good time to do this since you need to get into the capital cities for the Lunar Festival Achievement before the 14th.

 
Ain't no one getting the Violet Proto-drake until they've completed all the seasonal event Achivements including the Brew of the Year which doesn't finish until 12 months after it starts. So if you didn't pick it up last October, you won't be seeing this mount until Fall 2010.
The Red Proto-drake is quite a beauty as well, but requires you to complete a couple dozen Heroic instance WOW Mount Achievements .
The Plagued Proto-drake is another one that looks good on paper, but the Achievement involved requires you to defeat all the Wrath bosses the most difficult way possible, including this little gem.
You think that one was tough? The Black Proto-drake is even more fun. Tackle the all the Wrath bosses, but on Heroic mode. Good times.

Class Mounts
The two class mounts for Warlocks, Paladins and Death Knights count toward the total for this achievement. You will have to quest for them still. Sorry Druids, flight form doesn't count.

Dropped Mounts
These are an option, but you are facing the random number generator and we all know how that goes. Fortunately, there are good twenty to go after. And I hear the Baron is bit more generous than the days when we used to take down for his pants.

Seasonal Event Mounts
There's the Brewfest Ram/Kodo and the Hallow's End dreadsteed. If you don't want to wait until October to finish this Achievement, you may want to skip these. The temporary Flying Broom won't count.

Specialty Mounts
The Swife Zhevra mount from recruit a friend and the polar bear from the BlizzCon '08 card will also count.