So far these have been confirmed on 5.4 PTR
90, Mount, Reins of Galakras, Glory of Orgrimmar raider Achievement
90, Mount, Reins of the Kor'kron War Wolf, award from killing Garrosh final Boss
90, Mount, Kor'kron Juggernaut, Drop from Garrosh (either heroic only or rare drop like blazing drake previous expansion)
90, Mount, Reins of the Thundering Onyx Cloud Serpent, Drop from rare NPC houlon (low drop)
90, Mount, Tyrannical Gladiator's Cloud Serpent, PVP Reward season 13
90, Mount, Grievous Gladiator's Cloud Serpent, PVP Reward season 14
90, Mount, Prideful Gladiator's Cloud Serpent, PVP Reward season 15
90, Mount, Heavenly Golden Cloud Serpent, 100,000 Timeless Coins
90, Mount, Skeletal Warhorse for the Horde, PVP reward for 40 rbg wins and 100 arena wins achievement (horde only)
90, Mount, War Night Saber for the Alliance, PVP reward for 40 rbg wins and 100 arena wins achievement (alliance only)
90, Mount, Reins of the Ashhide Mushan Beast, 500 Bloody Coins
90, Mount, Hearthsteed, Promotion for Hearthstone
90, Mount, Sky Golem, Engineering recipe daily craft
90, Mount, Greatfather's Riding turtle, (rumor of festival of Winter Veil drop)
90, Mount, Moonfang, (possible drop of Darkmoon fairie world boss)
Not confirmed info
Boss Ordos Mount?
Celestial Bosses Mount?
Maybe Mount store, Raf promotion, Anual Pass, or purchased reward for new expansion or games. Who knows?....
Fairie Dragon Mount
If I missed any, please posted.
So far no info has been said of any mount being obtain by archeology or fishing even though beginning of the expansion there was a possiblity of that coming up.
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INformation may changed as we get closer to the live patch.