No longer obtainable in-game, maybe outside of BMAH.
This mount was a reward for the first ever Glory of the Raider achievement in 25-man revamped Naxxaramas, which in turn required Immortal achievement back then. Reward was removed from the achievement with release of Ulduar.
10-man version: Reins of the Plagued Proto-Drake
No sign of the mount yet on the Black Market Auction House.