Place exactly two cards named Hydra Head onto the battlefield from the Hydra's graveyard. If you can't, reveal cards from the top of the Hydra's library until you reveal a Head card. Put that card onto the battlefield and the rest into the Hydra's graveyard.
If there are five or more Heads on the battlefield, tap two of them and they don't untap during the Hydra's next untap step. Otherwise, reveal the top card of the Hydra's library and the Hydra casts that card.