Quote:
Currently some mounts do not have a corresponding opposite faction mount. In the case of a faction transfer where one of those mounts needs to be replaced a random mount is assigned.
This is basicly the general answer I found. Lots of people saying that the Alliance being cheated out of 2 mounts is bull so maybe they'll add two furry mounts, like two new horses or something just so they can balance it.
Some say that if you already have all the faction mounts the mount is just deleted, no word on if you have to buy it again should you xfer back. Others are simply crazy and say that you could get ashes or invincible.