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 their 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 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.