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.

If a player has no cards in their hand and Dinrova Horror returns a card to that player’s hand, the player must discard that card. They won’t have the opportunity to cast that card (or do anything else with it) before discarding it.

If the target permanent is an illegal target when Dinrova Horror’s ability tries to resolve, the ability won’t resolve and none of its effects will happen. No player will discard a card.