Polymorph

Not to be confused with shape change.

Possible forms

The caster of a polymorph spell must have seen the creature they desire to turn the target into. For example, a magic-user cannot polymorph someone into a white dragon if the magic-user has never seen a white dragon.1) We find this reasonable on its own, but a book precedent can be found on DMG 45. We can extrapolate from those notes 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.”

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 another elf will have 1 arm. That same 1-armed elf polymorphed into a spider will have 4 legs missing on one side.2) 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.3) PHB 78. “Creature” can be assumed to mean “living creature” and not “corpse”. Furthermore, the spell description heavily suggests a living creature.