Does a character need to know a creature to use Polymorph?
If yes, is a little information found in an old book enough, or does it have to be an encounter with said creature? If no, does the character know everything about all creatures he is able to polymorph into, including Resistances and abilities?
Is it something in the middle like: Hey I learned Polymorph, now I know all the beasts out there, but I only know their shape and size and nothing about abilities etc...
Thanks for your help
The description of the spell says:
The new form can be any beast whose challenge rating is equal to or less than the target’s.
Beast is a defined type of creature within the game, as per page 6 of the Monster Manual:
Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.
A reasonable interpretation is that the PC needs to have knowledge of the beast.
But how much knowledge? It seems reasonable that the caster can turn the target into a frog, even if the caster is not a frog expert. The caster may not even know much about frogs at all, except maybe that they hop. To my mind, it seems reasonable that the magic fills in the gaps. That if frogs say ribbet, then the transformed creature probably says ribbet, too, even if the caster doesn't know that frogs say ribbet.
Regarding your question about a little information in an old book, that is an interesting question. What if the old book is wrong? Old books are often filled with creatures which never actually existed. What if it is a new book? What if the book is fiction? What if the book is fiction and the caster wrote it? What if the book is fiction and the caster wrote it just now?
In the end, it is up to the players, particularly the GM to determine whether what the caster intends fits within the definition of "any beast".
As a GM, I would tell the player, that's an interesting idea, I suppose you could give it a try if you wanted. I would let the player use any source of in-game information, including the character's own imagination. I would also expect the character to be able to hold the concept of the target creature in its mind and that the player be able to state the target fairly succinctly. After all, it has a casting time of one action. If the intended results were out of balance with the power of the spell, I would modify those results. "I transform the big bad wolf into a cute kittycat", might work pretty well, at least the kittycat part. "I transform the big bad wolf into a piggy" would probably work. "I transform the big bad wolf into a flying piggy" might even work. Of course, if it were a chaotic evil wolf inclined to eat first and ask questions later, then the kitten, piggy, and flying piggy, would all be chaotic evil inclined to eat first and ask questions later, too. Having a face full of avian wild boar might not have been what the caster intended. The more absurd the caster's intention, the more absurd the results would be. That seems like rules-as-fun.
Polymorph spell (PHB p.266)
This spell transforms a creature that you can see within range into a new form.
The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.
RAW, there is no restriction on the beast form excepting the Challenge Rating allowed and the caster's ability to choose it. For a person to make a choice they need to know the choice exists and so that is all that this spell requires, the rest is handled by magic.
The important thing here is the difference between what the player knows and what the caster, the character, knows. It is the character that needs to know the about the beast form. The DM will be the final judge on what beasts the character knows about, particularly when a player pushes the limits of the spell, and they will.
For example: turning someone into a frog. This seems entirely reasonable (baring exceptional circumstances) as knowing about frogs is commonplace, even if you have never seen one (e.g. because the character comes from a desert setting that has no frogs). The frog is quite possibly an example of what the spell can achieve written in in "arcane manuals" or even the spell itself given the "turn him into a frog" trope.
However, as a second example, turning someone into an Ankylosaurus is going to take justification, in my opinion. Knowledge of the existence of dinosaurs, let alone specific dinosaurs, is quite possibly very rare, but again setting dependant. So while the player may have read the Monster Manual, the character has not and would have to have knowledge from some in-character source to be able to say "Siiiiiize of a Triceratops!" as part of their spell.
Do you need to know the creature to use Polymorph to change someone into that creature?
YES, the character needs to know about the creature to be able to choose it and it is down to the DM's judgement whether they do.
Of course the character casting the spell may know the beast form but may not have a good idea of the beast form's strength (the players will know this as its CR) compared to the target.
When it comes down to it this is a role-playing issue, where the player of the casting character has a responsibility to be reasonable and come armed with good in-character, good story, reasons to be able to use the beast form they want to use.
The spell description describes taking on the statistics of the "chosen beast." That would seem to imply that:
You might think of the new form being instantiated based upon a "platonic ideal."
Allowing flying piggies definitely seems like more fun though. Lots of opportunities for things to go slightly haywire there.
Everyone else already went into a deep dive into what they believe the spell means, so I'll keep mine short.
My answer is no, the player doesn't have to know the creature exists, nor does the player have to have seen the beast before.
In the first sentence of Wild Shape, it gives you the limitation that the player has to have seen the beast before.
Nowhere in the five paragraphs of Polymorph, does it states this, so you do not need to have seen it.
I'd even venture to say you could create your own beast, but then it wouldn't have a stat block which ends up putting more stress on your DM to come up with one.
