Animate Dead, Replenish, and a Enchantment Creature card
You choose what to attach the Dead to as it would enter the battlefield. Since the Courser is still in the graveyard at that point (it's not on the battlefield, so it's still in the graveyard), it's an eligible creature to attach the Dead to. However, right after Replenish is done resolving, before the Dead's trigger can even go on the stack, the Dead will go to the graveyard since it's not attached to a creature card in a graveyard.
Nathan Long
Magic Rules NetRep