Zombie Yeti

The Zombie Yeti is a very rare zombie that appears for the first time, after finishing Adventure Mode at least once, in the Fog level 4-10 (although, it doesn't contain any fog) in which it is raining and the player can only see when lightning strikes. If the player doesn't kill it in time, it will run away (with no reward) making it hard to kill. However, if it is killed, it will drop four diamonds for the first encounter but only 3 diamonds every time after that. From that point on, it will appear randomly in other levels (including Survival/Minigames). It cannot be chosen as a zombie in the I, Zombie puzzle mode. He's the 266609898th Zombie (20th based on the Suburban Almanac) you will encounter in Adventure Mode.

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).

Overview
Absorbs 75 Normal Damage Shots. Appearance Changes after 45 Normal Damage Shots before dying at 75 Normal Damage Shots. Will begin to run away after 12 seconds of being active on lawn. May also begin to run away after reaching the 5th square on lawn.

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 such as the Pumpkin or Tall nut. If that proves insufficient, plants capable of firing backwards such as the Split Pea or Starfruit can be planted in front of it or an instant kill can be used.

Gold Farming
One technique is to repeatedly attempt Level 4-10, the only level in the game where the Zombie Yeti consistently appears. Because the Zombie Yeti, when defeated, yields four diamonds ($4000), the gold farming application is obvious. Do note, however, that unless your version of the game has the Quick Play mode on it, (from which you can access Level 4-10 at whim), it is necessary to reach that point in Adventure Mode, after defeating it the first time. To be able to keep replaying the level, it is of utmost importance that the player does not collect the money bag at the end of the level, or restarts as soon as the Zombie Yeti is killed.

The Game of the Year edition does not allow farming of this zombie, because you can't Yeti farm by restarting level 4-10. Although if you restart and you don't kill him, he will still appear.

Trivia

 * The Zombie Yeti is the only zombie that will not appear until after you have finished Adventure Mode.
 * It is the one of the few zombies that can appear without being shown in the seed selection.
 * The Zombie Yeti and the Gargantuar (and its variant, the Giga-Gargantuar) are the biggest zombies.
 * The Zombie Yeti and the Zombie Bobsled Team are the only zombies that have the word "Zombie" at the start of their names, as well as the zombie.
 * In the Mini-game Big Trouble Little Zombie the small version of this zombie can appear but it does not drop diamonds.
 * The Zombie Yeti and the Digger Zombie are the only Zombies capable of walking in reverse direction without being hypnotized.
 * It is rare, but possible to see two Zombie Yetis in one level.
 * The Zombie Yeti and the Newspaper Zombie (once its newspaper has been destroyed) are the only zombies whose pupils are red.
 * The Achievement Cryptozombologist is unlocked by seeing it (Zombologist for Game of the Year, iPhone, iPad, iPod Touch, Android tablets, and Android phones).
 * 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 that Dr. Zomboss wears shoes).
 * The Zombie Yeti has eight visible teeth, the most compared to other Zombies.
 * It is also the only zombie with sharp teeth.
 * The Zombie Yeti, the Zombie Bobsled Team (except for Bobsled Bonanza), the Backup Dancer, Dr. Zomboss, and the Flag Zombie are the only zombies that can appear in a level without being shown in the seed selection.
 * If you hypnotize the Zombie Yeti, it will turn left after biting zombies. While it can lose its head, it does not drop diamonds when hypnotized.
 * It will be able to reach your house when this happens.
 * When you see the Zombie Yeti in Wall-nut Bowling, it takes one hit but still leaves the screen when not killed.
 * The Zombie Yeti costume is available on the Xbox Live Marketplace for 320 Microsoft points.
 * The Zombie Yeti, the Bungee Zombie, the Digger Zombie (if it reaches the left side of the lawn), and Dr. Zomboss are the only zombies that don't target your house.
 * Although the Zombie Yeti does not appear in the seed selection (with the exception of Level 4-10), it still has a standing animation in the Suburban Almanac.
 * The Zombie Yeti doesn't bend its elbows or knees when walking.
 * The Zombie Yeti, along with the Football Zombie, the Imp, and the old Dancing Zombie are the only zombies with normal feet (excluding the ones that don't step on land).
 * It is possible to encounter the Zombie Yeti more than once in a single Survival level, although it will only appear once every flag (or every two flags if in Survival: Hard or Survival: Endless).
 * The Zombie Yeti is one of nine zombies that cannot be used in Versus Mode. This is most likely due to the fact that it doesn't stay on the yard long enough to do much damage, if any at all. Other non-usable zombies include the Zombie Bobsled Team, the Imp, the Balloon Zombie, the Backup Dancer, the aquatic zombies (i.e. the Snorkel Zombie, the Ducky Tube Zombie, the Dolphin Rider Zombie), and Dr. Zomboss.
 * The Zombie Yeti, the Balloon Zombie, the Digger Zombie, the Flag Zombie, the Dancing Zombie (and Backup Dancers) the Zombie Bobsled Team, the Dolphin Rider Zombie, the Snorkel Zombie, and the Ducky Tube Zombie are the only zombies not to appear while battling Dr. Zomboss.
 * The Zombie Yeti is the only zombie with fur.
 * The achievement, Zombologist, calls the Zombie Yeti a "Yeti Zombie".
 * The Zombie Yeti is one of the three zombies to appear in the Windows Phone 7 trailer of the game. The others are: Imp and the new Dancing Zombie.
 * It is the only zombie which always drops a kind of prize.
 * The Zombie Yeti is the only zombie based on an animal.
 * This does not include the Dolphin Rider Zombie since the Dolphin Rider itself isn't an animal. But the dolphin is a zombie.
 * The Yeti zombie is the only zombie with sharp claws and has no shoes