Silver Covenant Hippogryph
As only Alliance players can become exalted with the Silver Covenant, this is correspondingly an Alliance-only mount. The Horde counterpart is the mount.
The Hippogryph is the adopted mount of the high elves after they separated from the rest of blood elven society, this is why the Silver Covenant (a high elven faction) have different mounts than their rivals, the Sunreavers (a blood elven faction, who have retained the use of Dragonhawks.
Although Hippogryphs are an intelligent species that gave their allegiance to the night elves, the reason why the Silver Covenant high elves use them have not been explained.
Patches and hotfixes