Destroy target creature. It can't be regenerated. Its controller reveals cards from the top of his or her library until a creature card is revealed. The player puts that card into play and shuffles all other revealed cards into his or her library.
"Ahh Opposable digits"
86 6E • ENRobert Bliss
™ & © 1993—1999 Wizards of the Coast
Legal in: Modern,Mirage Block,Legacy,Vintage,Freeform,Prismatic,Tribal Wars Legacy,Singleton 100,Commander
Oracle Text (click to copy):
View this MTG card on Gatherer
10/4/2004 If there are no creatures in the player's library, then the target creature is still destroyed, you see all the cards in that player's library, and then they shuffle and continue play. 10/1/2009 If the targeted creature is an illegal target by the time Polymorph would resolve, the entire spell is countered. Nothing else happens. 10/1/2009 If there are no creature cards in the player's library, all the cards in that library are revealed, then the library is shuffled. (The targeted creature remains destroyed.) 10/1/2009 A "creature card" is any card with the type Creature, even if it has other types such as Artifact, Enchantment, or Land. Older cards of type Summon are also Creature cards. 7/1/2013 If the targeted creature has indestructible, it's still a legal target -- it just isn't destroyed. The rest of Polymorph's effect happens as normal.