Zombie Yeti

The Zombie Yeti is a rare zombie that appears for the first time in the Fog level 4-10 (although it it does not technically contain any fog) after playing Adventure Mode through at least one time. If the player doesn't kill it in time, it will run away with no reward. However, if it is killed, it will drop five diamonds the first time and four for every other encounter. From that point on, it will appear randomly in other levels, but will not be shown in the seed-choosing menu. It will also remain in level 4-10.

Overview
Absorbs 75 Normal Damage Shots. Appearance Changes after 45 Normal Damage Shots before dying at 75 Normal Damage Shots.

Suburban Almanac entry
 Zombie Yeti  A rare and curious creature. Toughness: high Special: runs away after a short while Little is known about the Zombie Yeti other than his name, birth date, social security number, educational history, past work experience and sandwich preference (roast beef and Swiss).

Strategy
Just do whatever you do to kill Buckethead Zombies (besides using Magnet-shrooms). If it tries to run away, it can be stopped by planting a Wall-nut or another defensive plant in its path. If that proves insufficient, plants capable of firing backwards can be planted in front of it or an instant kill can be used.

Trivia

 * The Zombie Yeti and The Gargantuar are the two biggest zombies (Dr. Zomboss does not count, as he is not actually big and the only thing that makes him appear so is the Zombot).
 * In the Mini-game Big Trouble Little Zombie the small version of this zombie can appear but does not drop diamonds.
 * There's a chance that you may see two Zombie Yetis in one level.
 * The Achievement Cryptozombologist is unlocked by seeing it.
 * Along with the Balloon Zombie (while it has its balloon) and Bungee Zombie, the Zombie Yeti does not have a burnt animation. When they are incinerated, their bodies merely turn black and disappear, unlike other zombies.
 * The Zombie Yeti is one of only three zombies (with feet) that don't wear shoes, the other two being the Jack-in-the-Box Zombie and the Imp. (This is assuming Catapult Zombies, Dr. Zomboss, and Zomboni all wear shoes.)
 * The Zombie Yeti has eight teeth, more than any other zombie.
 * This is the only zombie that can appear in a level without being shown in the seed selection. (not counting the zombies dropped by Dr.Zomboss' robot.)