If you enter a battleground while sitting on the mount

Loaned Wind Rider Reins – Item – World of Warcraft Horde
Loaned Gryphon Reins – Item – World of Warcraft Alliance

You will be able to fly for around 10 seconds making you able to cross the fence on Arathi Basin start, Isle of Conquest gates and more

You get this mount by talking to “Honest” Max