I have a player with a 7th level druid. I can't tell you how much I dislike them having ever-increasing numbers of animals, elementals, and plants that they can just change into. Under the druid's wildshape ability it says "The form chosen must be that of an animal the druid is familiar with" but doesn't say how the druid is familiar with the form.
I want the druid to have access to a good variety of forms, but not every form possible on the planet. How do I rule which is a familiar form from one that they don't know? It was suggested that they have to make a knowledge check DC 10+CR of the form they want. I have considered giving them something along the lines of sorcerer spells known which are forms they know.
Can anyone offer advice on how to help balance this out?
I found one collection of shapes they can get for 4th, 6th, and 8th levels.
I was asked if I was asking as a player or DM. Ideally any question I ask will work for both, but in this case I am asking as the DM.
Best Answer
The Familiarity Question is a tricky thing in both Pathfinder and 3.5e. It is one of the worst problems: it can be interpreted in a variety of ways that can help both the DM and the Player. But there are a few ways to handle this.
Note: Regardless of your solution, it is a good practice to have the Druid have the stats of the creature on hand. If they can't immediately produce the rules of the creatur they want to Wild Shape into, it ain't happening. And no, you can't flip through the books to find them or see something that would match the situation: either have the rules on hand or it's a no-go.
As a player, aside from the above there's something else you should do: Don't be a jerk. Don't turn into creatures that allow you to break the game or speed past the various challenges your DM set up for the party. This will not make you popular with them and the first target for the assassin.
Also, put some effortinto it. Why does your Druid know all these monsters? Where does the knowledge come from? Have they traveled the world? Is their knowledge scholastic? Are they the scion of a noble (or even royal) house that owned an extensive zoo, allowing them to observe these creaures? Had an uncle who wrote The Great Manual of the Monsters of the Wild? Or is it something else?
As for the DM, you can add in rolls to see if the player knows the creature in question. The problem with this is that the role (when a regular Knowledge check) will become trivial at high levels, or perhaps render the Druid unable to Wild Shape into a common animal. And if you use custom rolls, have fun determining a proper DC for every monster.
Another option would be to enforce a hard limit (you know X creatures you can change into) per level, but this is very arbitrary.
Though in your situation, I think your best option would be to look at the terrain. If you open the Bestiary you will see that every monster has a little writeup to show its type, its terrain and climate. By using this you can determine where your creature is from. Then look at where the Druid is from. It is logical that a Druid who lived in the cold mountains all their life has never seen a camel or a crocodile, or that a Druid from the heart of the jungle has never seen a dolphin. Use this and determine a number of terrains that match the character in question. Then if they want to Wild Shape into a creature, match its terrain and climate to that of the Druid. Is that where they are from? You can turn into that creature. If not, they can't use that form.
Of course, this requires some reading to see which terrains have a usable list of creatures. Perhaps you can use several combinations so that the Druid has some creatures to choose from, but not all of them. And of course, have it make sense. You could link for example temperate forests, hills and mountains together, or perhaps the shores of a desert and combine them with the urban and water environments.
Whatever you choose, do share this with your players in advance. It builds expectations and allows them to make a choice that fits your house rules.