Destroy target creature. It can't be regenerated. Its controller reveals cards from the top of their library until they reveal a creature card. The player puts that card onto the battlefield, then shuffles all other cards revealed this way into their library.
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 doesn’t resolve. 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.