For a apear: a short range "throw spear" spell so a normally melee-only unit could moonlight as a ranged one.
Tower shield: Shield wall. Increase defense, reduce offense
Smaller shield(s): Shield Bash. Small chance to trip/stun opponent.
Warg: Maul (seize 'em!)
Horse: Cavalry charge. I'll be damned if I can figure out a way to make it useful but it sure sounds cool. The best implementation of a charge I have seen in Shadow of the Horned Rat. A cavalry unit had to go in a straight line for several squares before it became "a charge".
Elemental: To move up to the enemy, stop, and then click "charge" would be dumb.
You can't just make it expensive "so the unit needs full movement points" because even 0,1 points are enough to activate any ability at all.
The effect of a charge would be easier to come up with. Something to reduce retaliation/counterattack, possibly stun the opponent or reduce morale.
I'm afraid that any "cool" way to make it work would have to wait until things can get scripted properly.
Then cavalry units could become very cool but they would need maneuvering space to line up their charges...