Zombie Yeti

The Zombie Yeti is a rare zombie that appears in the Adventure Mode level 4-10 (The Fog conveyor belt level) after playing Adventure Mode through at least one time. If you don't kill it in time, it will run away, but if you do beat it, it will drop 5 diamonds (four in later encounters) and from then on randomly appear in other levels. (It will also remain on level 4-10.)

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).