Skeleton Horses and Zombie Horses were changed to undead in 1.9, but in 1.7 they're still not, which for example means that they take damage from damage potions and are healed by health potions.
Giant Zombies are also not flagged as undead, though vanilla never fixed that.
All 3 of those mobs should be undead.
Skeleton Horses and Zombie Horses were changed to undead in 1.9, but in 1.7 they're still not, which for example means that they take damage from damage potions and are healed by health potions.
Giant Zombies are also not flagged as undead, though vanilla never fixed that.
All 3 of those mobs should be undead.