Mountain O' Mounts - Complete Help Sheet!

[myrcia]

[Little White Stallion Bridle] does not count for this achievement. It's a consumable mount.

[R11tz0r]

Nice guide m8, you'r only missing "Onyxian Drake" that is Onyxia itself.
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[Phoenix] - Kael'thas Sunstrider - Magister's Terrace **RARE** (This is completely wrong) It's named "Ashes of Al'ar" and drop from Kael'thas Sunstrider in "The Eye"(Tempest Keep)
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The Resonating Crystals DO count towards the Achieve!
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Celestial Steed bought from Blizzard Store.
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Reins of the Crimson Deathcharger (Extremely Rare - When you kill the Lich King with a player wearing Shadowmourne in the raid, you will get an additional item - Unsealed Chest. After bringing the chest to Darion he will open it up and let you bring the items back to their respective owners to get your rewards :

Reins of the Crimson Deathcharger - A brand new mount that looks like the Death Knight's class mount.
Muradin's Favor - A 10-minute Frost Dwarf transformation.
Jaina's Locket - A portal to Dalaran on a 1-hour cooldown, you finally have your own pocket mage!
Tabard of the Lightbringer - A tabard with a very shiny on-use effect.
Sylvanas' Music Box - A music box that plays Lament of the Highborne. (Quoted from wowhead)
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Mimiron's Head (Extremely Rare - Drop From Yogg-Saron)