Willow

 is a tree monster. Once a tree fairy had lied within but it seems the fairy's gone crazy now. The fairy had endowed a little magical ability to the monster though... Creature reborn from an old tree. The feature and the sounds it makes are incredibly eerie.

Ragnarok Online II
On RO2, there are 3 varieties of this monster:
 * Willow
 * Moss Willow