Polymorph (Any)
There are several ways for creatures to be transformed to an entirely different species: Wand, Ring, divine punishment, or otherwise gaining the Polymorph intrinsic (likely by eating a Chameleon corpse). The Unchanging intrinsic will prevent polymorphing altogether, regardless of cause.
In most cases, a polymorphed creature will become a random monster type (no bosses, named uniques, or "super-rare" monsters), anything from a Mouse to an Iron Golem. However, Polymorph Control will allow a creature to choose their new species -- the avatar can choose from their original form, or any species which they've previously defeated (even the super-rare monsters, or a boss you've beaten already). Exception: The divine punishment will turn you into one of a few wimpy monsters. You cannot "control" a divine punishment polymorph, but Unchanging will still prevent one.
When polymorphed, the creature will gain all the abilities and intrinsics of the new form, but cannot use their original spells or skills. (That can be a problem if you had Endure Hunger and were running hungry!) They can, however, learn new skills or spells, which will be kept when they return to their original form. The new form may have a different set of equipment slots -- any item equipped in a now-missing slot will fall to the floor (or water, etc.). Note that no monster has more slots than the original human set! In particular, the various Tridude types don't have three hands or three ring-fingers -- indeed, their only slot is their head.
Once polymorphed, the creature has a 1 in 1000 chance per turn of spontaneously reverting to their original form (Unchanging will prevent this). If the new form is killed, they will revert to their original form and suffer System Shock. If the "slain" creature has Life Saved, that will take effect first (they will not revert). If they have Unchanging, they will not revert, but simply die.
Some temporary intrinsics will be cleared by polymorphing, and/or by reversion. If the creature was poisoned, the new form will not be poisoned, but when they revert, the poison will return with their original form. Experience will be retained across forms, as will food level (hunger). (Of course, if the new form needs no food, or has the Endure Hunger skill, their hunger level will not matter.)
Note that the experience you receive for kills will be figured based on the "level" of your current form. So it may be fun slaughtering things as an Iron Golem, but you won't get much experience for it. (That Non-Regeneratable thing is also a bit of a drag!) On the other hand, it may be worthwhile for a powerful character to polymorph into a weaker monster that will get more experience from kills.
Objects can be transformed only by a Wand of Polymorph (or, rarely, by a punishment of Xom). This can transform only one item at a time, even from a stack, though a weapon might be transformed into a stack of arrows. Items are changed to another item within their class. The holy/evil status, plusses/minuses, and wand charges are all figured afresh for the new item -- the old values are ignored. This cannot transform artifacts or an Amulet of Unchanging, and the new item will not be an artifact (nor an elemental weapon). Corpses transform into empty bottles.
