If you spawn in a zombie horse (/summon EntityHorse ~ ~ ~ {Type:3}) or a skeleton horse (/summon EntityHorse ~ ~ ~ {Type:4}).
Then throw splash potions of health and damage and you will see that even though health and damage potions work reverse with other undead mobs (like zombies and skeletons) these potions will have the ordinary effect on the undead horses as if they weren't undead. an other way to reproduce this bug is to spawn a wither, and you will observe that the wither attacks the undead horses.
Yup that's because zombie and skeleton horses have not been fully implemented. You'll notice you can't ride them either.