Instructions
Open your "World of Warcraft" folder, which is the folder you specified during installation. In most instances, this is in the "Program Files" folder on your computer's "C" drive.
Right-click the "cache" folder. A right-click menu will appear beside your cursor.
Click "Delete." Confirm your selection. The entire folder will delete and "World of Warcraft" will automatically generate a new one the next time you load the game. The mount will also appear without any problems.