Black Battlestrider

Black Battlestrider is an epic PVP mount. Previously available only to gnomes and dwarves who had reached a PVP rank of Commander (rank 11), this can now be purchased with Marks of Honor and, as of patch 3.0.8, can be mounted by races other than dwarves and gnomes. Before The Burning Crusade, this mount only required Riding Skill 75 and level 40; it now requires Riding skill 150 and level 60.

Source
This item can be purchased from just outside the door of the Champion's Hall in Stormwind as part of the PvP honor system Rewards. It costs.