I don't understand why mounts give initiative bonuses; why can you attack or cast faster while mounted? Weight capacity makes sense, and may influence initiative that way, but as a direct modifier it isn't very realistic. Movement makes sense, dodge makes sense, prone resist makes sense (though something less than 100% might be nice).
What about combat bonuses for having higher ground?
I see horses as being more practical/solid, and wargs as the more agile option. So, off the top of my head...
Horse: +2 move, +20 carry weight, +5 dodge, +2 to hit, +1 attack, +1 defense, +75% prone resist, +10% tactical mana cost
Warg: +2 move, +10 carry weight, +10 dodge, +4 to hit, +1 attack, +1 defense. +50% prone resist, +25% tactical mana cost
As pointed out, the movement bonus is still pretty huge (especially out of combat). But making Animal Husbandry a prerequisite for horses (and by extension, wargs) also makes sense from a flavor standpoint - and increases the cost, as well. Upping the gold cost isn't a bad idea, either. Is there any way to give equipment (a mount) an upkeep cost?