Polymorphing transforms items into diferent items and your character or monsters into different monsters. For example, a rock can be polymorphed into a scroll, and a human can be polymorphed into a buffalo. There are various methods how to polymorph, and how to control or prevent it.
Several Status Effects are associated with polymorph.
An uncomfortable uncertainty of who you really are overwhelms you.
Polymorphitis is a state that forces the character to polymorph at random.
If you do not have a means of polycontrol, the monster you transform into will also be random. If you do have a means of polycontrol, you will be prompted to select a form to take whenever your polymorphitis triggers - see below.
Polymorph lock is a rare state that prevent you from polymorphing - or from reverting to your human form if you gain it while already polymorphed.
It can be gained from a ring of unchanging.
Polycontrol is a state which allows the character to choose which creature they polymorph into. The easiest way to be permanently in this state is to wear a ring of polymorph control. You can also temporarily have polycontrol by eating a Magic Mushroom or drinking from a fountain. Wielding Justifier or Y'yter Durr will also bestow it on the character.
Whenever you polymorph with a polycontrol active, you will be prompted to choose which creature to polymorh into. Pressing ESC or choosing human will cancel the polymorph, as you are human by default. Otherwise, you have to have seen the creature at least once in the given playthrough to be able to assume its form, as well as your Intelligence have to be sufficient. See individual creature entries for PolyControl requirements.
It is not possible to control polymorphing of items.
Wand of Polymorph
Zapping a wand of polymorph will polymorph any objects or creatures within a 5 block range. Up to five items on each block (the five closest to the ground) will be polymorphed. Some items cannot be created through this method, such as scrolls of charging. Some creatures, mainly the named creatures, cannot be polymorphed.
Applying or otherwise destroying a wand of polymorph will polymorph any objects or creatures in a 3x3 grid centered on the wand.
Also see Polypiling for more information on polymorphing items.
A Chameleon whip will sometimes polymorph the creature it hits.
Drinking from fountains, stepping through Raw Vapourized Magic (released by Magic Mushrooms) or Mysterious Red Gas (released by Staff of wondrous smells), or eating mutant flesh can possibly give you PolyControl and/or Polymorphing Status Effects.
The only creature that polymorphs on its own is the Chameleon. Chameleons will polymorph when they feel threatened, and can polymorph into any non-unique monster.
Many unique NPCs are outright immune to polymorph, so that you cannot accidentally prevent yourself from making certain quests impossible to complete.