[Endoperez]

It probably wouldn't be that big of a chance to change their mounts into normal horses. I'm afraid it's hard to balance them properly, though - they are essentially light cavalry, and light cavalry sucks, and I can't make them knights because they weren't. I'm trying to make a unit that can compete with Centaur Warriors or the EA non-sacred mounted Vanir units, except that it can't. It's not sacred, it doesn't berserk or have glamour, and at least the Centaurs are cheaper. The Mounted Hirdmen might be 55 gp or so - 15 gp isn't nearly enough for such a huge difference in power.

Hiisi is way different from Tapio. Both the race and the king are called Hiisi, and they have nothing to do with the forest apart from living in the wilderness and avoiding cultivated ground.
If I were to implement them, Hiisi would be Vaettir for the small ones and either Jotuns or Trolls for the bigger ones. There's a story in Kalevala about hunting down the "hiien hirvi", the hiisi's great elk (moose)... and both Vaetti and Trolls use mooses in warfare. I probably won't make them recruitable, but that's one thought.