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.
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Follow this map.
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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.
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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.