From now, if you have Animate Horse summoned:

If your summon is attacking a target and has aggro, if you cast Animate Dead, the skeletons will join forces with your Horse and attack the same target until the target is dead or for 30 seconds, they will then turn on the nearest target.

If you don't have Animate Horse summoned, Animate Dead will stay as it was with a 60 seconds cooldown and attack nearest target.