Zombie cavalry is a formation of zombies that consists of zombie horseman.
Zombie horseman wears plate mail and shield, moves as fast as horse, and charges with reach3 awl-pike. It can leap over walkable obstacle including trap. Fire, pit, caltrops, and acid, only damages the horse but not the rider. When the horse is killed, the horseless horseman walks slowly, but is still formidable. When a zombie horseman is killed before its horse is, it turns into a galloping zombie horse brute with plate mail, and doesn't follow formation.
With the speed, damage, armor, weight, and number, zombie cavalry requires tactics to deal with.
The existence of zombie cavalry needs thematic reason, though. It could be a player triggered event that happens in temple, museum, or other reasonable places.