A probability to summon an Ent when you cut a tree, why not a dangerous creature on surface, not as powerful as a Troll, but powerful enough.
Low probability to spawn one, low speed, powerful ranged attack to not outrun them, and a big hit on melee, it doesnt sound too fantastic for me to ad it into the game, the power of the Ent can be determined by the size of the tree, like a pine that drops 3 logs when it is cut down.