When Dinrova Horror enters the battlefield, return target permanent to its owner's hand, then that player discards a card.
3/14/2017 If the target permanent is an illegal target when Dinrova Horror’s ability tries to resolve, the ability will be countered and none of its effects will happen. No player will discard a card.
3/14/2017 If a player has no cards in his or her hand and Dinrova Horror returns a card to that player’s hand, the player must discard that card. He or she won’t have the opportunity to cast that card (or do anything else with it) before discarding it.
3/14/2017 A token permanent returned to a player’s hand isn’t a card and can’t be discarded. It will cease to exist when state-based actions are performed after the ability finishes resolving.