On another thread I posted this. This is what I think the mounts system should be like.
How about this, once you learn a spell or get a special summoning treasure card you can then use some mana ( 50 or so would seem reasonable ) to summon that creature outside of battle and then can ride it. Of course, creatures such as the storm shark would be summoned in water. This could also encourage teamwork and the creation of teams as one person may have a storm shark to get across a lake inside of an instance, while the others may not, they can ride on the back of the shark at the same time as the summoner.
Seems like a really fun idea to me. What do you think?