One of my player knights wants to charge at beasts while hunting lance in hand.
Is that appropriate? What about charing from horse back with a boar spear? Would the appropriate skill be lance or great spear?

Oh the silly things these knights do...

I wouldn't permit a lance charge unless the terrain is relatively free of obstacles (open fields, hills, or barren wastes; NOT forest, wildlands, marsh, swamp, or mountains) and the target beast has a much lower movement rate than the knight's mount.

I'm with the Undead one. Imagine you are chasing a wild boar on horseback in an open field. You're not going to be able to get it, it's turning is much quicker and much sharper than your horse and would you want to risk your horse against it's tusks. Invaribly dogs and a team of people are used to corner the beast and then you can go in, on foot and test your mettle against it.