There should be a new mob that incorporates the zombie horse that has been sitting around in the code for a while now.
So basically when fighting a witch, the witch has a chance of cursing a player similar to how killing a pillager captain gives you the bad omen effect.
Once cursed the player will see the headless horseman spawn near them every night. The headless horseman will stop spawning near the player if they kill the headless horseman or if they drink milk to remove the curse effect. The headless horsemen if not killed will just disappear in the morning only to return the next night.
The headless horseman has the appearance of a zombie without a head or a pumpkin for a head and the zombie is mounting the zombie horse. The headless horseman carries an iron sword and does the damage of an iron sword.
On death the headless horseman drops rotten flesh and has a chance to drop a pumpkin or an iron sword. The zombie horse can be tamed and ridden.
The headless horseman has eleven hearts health
Like all potion effects the curse effect can be removed by drinking milk.