Yeti

It lives above the height where anything sensible lives and it has the whole mountain on its side. It does not need to be very clever or very fast, because the cold does most of the work, and the thin air does the rest, and it knows which slopes will go if something heavy crosses them at the wrong angle. Expeditions that meet one describe a short violent encounter. Expeditions that do not meet one and turn back anyway have usually made the better decision.

Fists: bludgeoning, melee 15 ft, +6 to hit, 3d8+5, twice each round, delivered by something that does not appear to feel the cold or the altitude. Bring the Slope Down: 60 ft, save Dex Quick, fail save roll 3d8+5 bludgeoning as a loaded face lets go above the party, and creatures that fail are buried and begin suffocating. Cooldown 3 rounds. It chooses where to fight with this in mind.

The Mountain Helps: at altitude, in snow, or on ice, every creature not native to it fights at Disadvantage and it does not. Immune to cold. Buried: a buried creature is dug out by one ally spending two turns, or by the buried creature winning a Strength Chance Roll against a TN the DN sets, and the yeti will spend that time on somebody else. Reduced to 0 HP it dies, and the party is still four days above the snow line with a body they cannot carry.