Yeti

The Yeti exists only inside a Frost Moon, and only once the night is well along, from the twelfth wave onward. That puts it behind Plantera by necessity, since nobody reaches wave twelve without post-Plantera gear. There is no summon for it and no way to meet one outside the event.

It moves like the Krampus and the Nutcracker, a ground charger that closes and swings, but it hits harder than either and knockback does nothing to it at all. If your plan for the night was holding the crowd at arm's length with a heavy weapon, that plan stops working the moment one of these reaches you.

Height is the answer. Stay a few blocks above the top of its head and it will not attack, which is why serious Frost Moon arenas put the player on a raised platform instead of on the ground. Each kill is also worth a large chunk of wave progress, so killing them fast is what pushes the night deeper rather than merely surviving them.

The yeti, called the abominable snowman outside the Himalayas, is the shaggy ape said to live above the snowline there.