Wanted to add this fact>. The person doing the Mentoring must be a higher adventure level than the Prospect and be riding this horse.
These can stack, if you have two people who are riding a mount Mentoring one person that person will get twice the bonus.
Once these mounts enter the game, they are no longer tied to any account status that is connected to RAF; they work with any player.
So from the sounds of it, I should be able to use my two old acnts and when my first team hits max level, i can group the two mains, mentor while mounted(4 noobs) and the 4 invited accounts will all get 3xRaF exp PLUS 20% from the two mounts? sweet!