So wait...
My paladin on alliance has like 110 mounts. My DK on horde has around 110 as well. However, my paladin has got like ~10 mounts that my DK doesn't have an vice versa. For example, my DK has Green Proto Drake but my pally doesn't. My pally has Swift White Hawkstrider, but my DK doesn't.
When MOP hits, will my Horde DK get the White chocobo and will my alliance pally get the Green proto? Or does it only give it per faction?