Not to be confused with shape change.
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) 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)
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 2 legs missing, one on either side.3)
Not allowed with polymorph other.4)
Polymorphed creatures do not yield items of value, such as skins (see Treasure List), nor can they be harvested for magic item components (such as scroll ink).Ex
Acts of reproduction committed while polymorphed will not result in the birth of new creatures.