DarkHorse 
DarkHorse is a simple, lightweight plugin that lets you spawn all types of rideable beasts in Minecraft, INCLUDING the "darker" horses in the game. That's right - zombie and skeleton horses! Using a simple set of commands, you can spawn in these mobs immediately and shock people with the unnatural horses!
And now, not only can you spawn all the types of horses, you can even ride them too! Spawn a tamed horse, right-click with a saddle, and travel through the countryside on your noble undead steed!