A commandable carrniverous horse with fangs (as in sabertooth) and claws instead of hooves. Give it limited flight capabilities with a fire animation for each foot and nostril. Animated wings (of fire) and armor would be optional. Make riding a mount a Skill with separate options for mounted attacks, trample and spellcasting. Note use a limited list of spells for self and ranged attacks or use a modified device such as the Staff of Lore but as a bataan ( as in Marshall's ) or wand. As a further limitation make Mounted Spellcasting possible only from the back of a magical creature. Being able to use a lance would be very good. How about an animation for a rearing horse as well. Oh yeah, make it look as good and with as many variations as Slofs Horses. Well, thats quite a wish list and I suspect just about impossible. However, one further note: With all of the mounts in the game I have wondered why no one has turned Riding into a Skill especially since the advent of the mounted combat and spellcasting capabilities associated with Deadly Reflex. Seems kind of obvious.
You could choose a mount(s) to specialize in and the kind of combat desired: Flight, Lance and Melee, and Magic.
Types of Mount Specialization:
Horse
Unicorn
Dragon (Flying)
Pegasus Flying)
Eagle (has no one invented that yet?!)
Elephant
Tiger
Nightmare
Trainers and a Riding school might make an interesting mod.
I apoligize if that's more info than what you wanted (blame it on half a pot of coffee