Dolphin

The Dolphin is the mount of the Dolphin Rider Zombie. According to the almanac, the dolphin is a zombie too. The Dolphin disappears after it jumps. Dolphins are very fast.

Strategy
The best ways to counter dolphins (or actually, Dolphin Rider Zombies) is to either plant a Tall-nut in front, which the Dolphin cannot jump over, or plant a Lily Pad in front to make the Dolphin Rider Zombie lose his dolphin (if it is planted in the first row, the dolphin will not jump). Then, kill the zombie normally.

Another way is to plants a tangle kelp in the dolphin rider zombie's path, therefore dragging it down to the bottom of the pool.

Trivia

 * In the music video Zombies on Your Lawn, the Dolphin said "I'm going to eat your brains!", but the dolphin was never seen eating brains in the game.
 * Also, when the zombies say "We are the undead!" it says "We are SO the undead!".