Balloon

Balloon is a red helium-filled balloon and has a face with angry eyes and fangs drawn on it. It belongs to the Balloon Zombie and is used to bypass the player's defense.

It is technically an armor as it gives the zombie a special ability and will take one hit for its owner before being popped. Having said the above, it is neither a shield nor headwear but can still protect its owner from a very small amount of damage, and thus belongs to a unique category of armor and is the only one so far belonging to its category.

The balloon's only weaknesses are spikes from Cattails and Cacti, which will pop it and cause the zombie attached to it to fall to the ground. If the balloon is popped over water, the zombie will instantly drown.

Trivia

 * In the DS trailer, Dr. Zomboss creates a zombie with a balloon for a head.
 * The balloon is also seen when you reach 1,000 points in the leaderboard, but its owner is nowhere to be seen.