Quel'dorei Steed

The Quel'dorei Steeds are unicorns used by the high elves of the Silver Covenant. They became available to players with Patch 3.2.

Source
When exalted with the The Silver Covenant, Alliance players can buy this mount at the Argent Tournament Grounds from for.

Mount Journal
Comes from a lineage of fine horses that once grazed near the shores of the Well of Eternity.