Fire elementals have a catastrophically low initiative for ranged units and a significant damage spread. Nevertheless, in my opinion, they remain the best elementals. As a rule, these are elementals summoned through the "fire warriors" perk. In the early stages of the game, they are incredibly effective.
As for Yetis, they are impossible to find in the game; the developers were in a hurry to release the game and forgot to remove them from the resources. But on some unofficial maps, they are implemented as a mod.
P.S. Please don't ask me which ones. Honestly, I don't know.
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal