Question: In trying to set reasonable limits on a PC's usage of the polymorph chain of spells. Is it plausible to require that the PC have some form of interaction with the target creature? Or can a PC polymorph into a creature that the PC has only heard about? Or read about in a book? Or seen just a painting of? From a solid role-playing standpoint, it seems reasonable to require that a) the creature exists in the multi-verse that the DM is running, and B) the PC has some knowledge of the creature. What is the minimum "knowledge" that the PC must have in order to polymorph into such creature?

Answer: 
In my opinion, you should at least **have encountered the creature before** to polymorph into it. You might even have to examine a body. Reading about it in a book or rolling Knowledge should only let you recognize it.

1. It lets the DM prohibit creatures with abilities that are broken in the hands of PCs, or creatures which don't exist in his game world.
2. It adds realism. How can someone who's never seen an ettercap expect to replicate its form, let alone its poison glands?
3. If the player really wants a certain form, they can go and hunt down a creature of that type, and this can be the seed for an adventure.
4. It prevents the player from assuming the forms of high-powered, low-HD creatures that would normally be above their challenge rating. However, this is rarely an issue since creatures with low HD for their CR creatures tend to get their power from supernatural abilities that you don't get with *polymorph*.
Once I used a table based on the teleport tagert table, it was something like it:

```
Familiarity          Fails/Wrong Monster     Weak Version     Exact Monster  

Very familiar                                 1                 2-20       
Studied carefully     1                       2-3               4-20      
Seen casually         1-2                     3-5               6-20
Viewed once           1-4                     5-8               9-20 
Read or saw picture   1-8                     9-15             16-20
Monster doesn't exist 1-19                    20

```

With fails/wrong monster I would jut choose some monster or animal visually resembling the monster but very weak, or totally opposed thing, "So, let me see, you turned into a... Giraffe..."...

A weak version could be the same monster, but without power, or attributes modifies reduced, like instead of +8 in Str, you get +2, etc..

You could fine tune the checks using the CR of the target and the caster level of the character