Minecraft wiki entry for Zombie_Horse, associated text: Dr. Zhark appears in the credits after the End Poem as the creator of horses, including zombie horses.
If a player pulling a zombie horse with a lead enters a boat, the zombie horse swims after them at the same speed as the boat.
A player riding a zombie horse is dragged along if their zombie horse is being pulled by a lead, and can even be lifted up in the air in the same way.
The texture for zombie horses contains an unused saddlebag.
The Zombie Horse is the only one of the 4 unused mobs that has a spawn egg.