The first triggered ability isn't targeted, so the creature cards aren't chosen until the ability resolves. Each player chooses which creature card from his or her own graveyard to return. If a player doesn't have an applicable creature card, that player does nothing. If a player does have any applicable cards, that player must return one to the battlefield.

If this enters the battlefield in a way other than announcing it as a spell, then the appropriate mana can't have been paid, and you'll have to sacrifice it.