Polymorph | Magic: The Gathering
Formats Polymorph is Legal in
Official Oracle Text for Polymorph
Official Rulings for Polymorph
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.