This creation now exists in a Plants vs. Zombies game, but did not at the time of its creation. The official plant/zombie is Calvary Zombie and this creation was made on July 2011.
Cavalry Zombie is like a land version of Dolphin Rider zombies. But, instead of jumping over a plant, the horse will just squash them. If the plant is too tall or long (Cob Cannon, Tall-nut etc.), the horse will jump above it and squash it. If you plant instant kills at it, it will instantly squash them so that would give you a waste.
Even though projectiles seem to be hitting the horse, the rider is the one who's absorbing damage.
If a Cavalry Zombie moves over a Spikeweed, the horse will instantly stop and do that standing on its hind legs. Then it will turn back to normal.