Polymorph

Not to be confused with shape change.

Possible forms

The caster of a polymorph spell does not need to have encountered a creature to be able to turn a target into that creature.1) This is a common house rule but nowhere in the books does it say this is the case. The only related text is on DMG 45, on phantasmal force: “The magic-user must know of and understand the force/creature he/she is making an illusion of. Thus, if the caster has never cast a fireball or has never seen a dragon turtle, his illusion of such will be very poor.” However, phantasmal force is both an illusion (not an alteration), and lower level. So it is not sufficient precedent to restrict polymorph spells similarly. However, the caster must at least know something of the creature to be able to picture it in their mind. It can be assumed that adventurers will know enough about the monsters in the monster books to be able to polymorph into their forms. However, consider the situation where a caster is on an adventure and is warned by a traveler of hhgninflorfers in the cave ahead, the hhgninflorfer being a monster devised by the DM and who live only in that particular cave. It can be reasonably said that the caster cannot manifest a hhgninflorfer simply be hearing the word. Any restriction on the spell beyond the “hhgninflorfer example” is undue restriction of the spell.2) Further explanation of this ruling: There are cases in swords & sorcery literature where a spellcaster cast a spell without an understanding of the form they were manifesting. Sometimes the caster of the spell wasn't even a trained spellcaster. And in old-school D&D, magic comes from other planes, and we do not view it as a science like it is depicted in some fantasy. Magic in AD&D is more about channeling unimaginable powers from other planes. Sure, the caster might be carefully and precisely channeling it, but we don't believe that a magic-user is biologically forming, say, a dragon, through biology when they cast polymorph. They're using magic! They don't need to completely understand the form they're calling forth.

Body part restoration

Polymorphing can not restore lost limbs or other body parts. The polymorphed form will always have a proportionate amount of limbs/parts to the new form. For example, a 1-armed elf polymorphed into human will have 1 arm. That same 1-armed elf polymorphed into a spider will have 4 legs missing on one side.3) This ruling fits with the trope. It also prevents a 4th level spell from providing a higher level spell effect as a side effect. Yes, system shock is considered, but with high constitution, the risk is low.

Dying while polymorphed

See Death.

Polymorphing the dead

Not allowed with polymorph other.4) PHB 78. “Creature” can be assumed to mean “living creature” and not “corpse”. Furthermore, the spell description heavily suggests a living creature.