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Magic can cause creatures and characters to change their shapes—sometimes against their will, but usually to gain an advantage. Polymorphed creatures retain their own minds but have new physical forms.
The polymorph spell defines the general polymorph effect.
Unless stated otherwise, creatures can polymorph into forms of the same type or into an aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, ooze, plant, or vermin form. Most spells and abilities that grant the ability to polymorph place a cap on the Hit Dice of the form taken.
Polymorphed creatures gain the Strength, Dexterity, and Constitution of their new forms, as well as size, extraordinary special attacks, movement capabilities (to a maximum of 120 feet for flying and 60 for nonflying movement), natural armor bonus, natural weapons, racial skill bonuses, and other gross physical qualities such as appearance and number of limbs. They retain their original class and level, Intelligence, Wisdom, Charisma, hit points, base attack bonus, base save bonuses, and alignment.
Creatures who polymorph keep their worn or held equipment if the new form is capable of wearing or holding it. Otherwise, it melds with the new form and ceases to function for the duration of the polymorph.
Since creatures do not change types, a slaying or bane weapon designed to kill or harm creatures of a specific type affects those creatures even if they are polymorphed. Likewise, a creature polymorphed into the form of a creature of a different type is not subject to slaying and bane effects directed at that type of creature.
A dwarf’s bonus for fighting giants is based on shape and size, so he does not gain a bonus against a giant polymorphed into something else, but does gain the bonus against any creature polymorphed into a giant.