Tome of Polymorph: Serpent

Tome of Polymorph: Serpent taught a mage the new ability: Polymorph: Serpent. Learning the spell consumes the tome. This spell is mechanically the same as Polymorph (Rank 4), except the target is transformed into a serpent instead of a sheep.

Source
This was bought from Endora Moorehead in Dalaran for. This was a limited-availability item.