The Alterac Valley of Olde achievement from Korrak’s Revenge doesn’t currently reward any mounts on live servers.
According to multiple Reddit reports, completing the Alterac Valley of Olde no longer rewards the faction-specific mounts in the Dragonflight pre-patch. We do not know if this is intended or if Blizzard forgot to add them when they enabled Korrak’s Revenge on live servers.
The achievements previously rewarded faction-specific mounts — Stormpike Battle Ram for Alliance players and Frostwolf Snowler for Horde players.
Stormpike Battle Ram