The Yetis can appear in rare raids (Unanoounced) on Folda and Chyllfroest (Accessible between April 1 and May 1).
They spawn in small groups, from 1 to 3.
On Chyllfroest, they spawn very rarely on the fourth floor of the mountain.
Follow this map.
On Folda, one Yeti group spawns close to the shovel hole, second group spawns little bit north east of the shovel hole and the third group spawns east of the shovel hole.
The most easy place to found it is Folda. The best way is bring here a character (Mages better) with level 45+, get full supplies (Sudden Death Runes and Mana Potions) and check the respawn when you have time.