So one of my players is now playing a Druid and when he wanted to use Wild Shape, several questions came up:
1. How to establish which beasts the druid has seen before the adventure?
What sort of land is the Druid from? Look at the character's backstory. A Druid from a temperate area is probably going to know wolves, horses, bears, eagles etc, but probably not tigers, crocodiles or giant snakes for example. A Talenta Halfling probably knows Pteranodon, Velociraptor, Deinonychus etc forms, but not horses, dogs or big cats etc.
Get together with the Druid player and make a short list of forms that the Druid knows, with maybe 5 or 6 options covering a variety of bases with stats ready-written out. Let them know other standard forms as well though.
Let the druid spend downtime getting to know new beasts that are in the area that they are in at the moment.
2. There are a bunch of beasts in the PHB, but what about creatures that are only in the MM. Players shouldn't know their stats, right?
There is no difference between knowing the stats of creatures in the MM compared to those in the PHB. A Druid will probably know several forms from the MM.
Players using the out-of-character stats in either the MM or PHB to metagame is to be discouraged. Pulling out the book and looking up stats as soon as they run into an encounter involving specific creatures is pretty much as rude as a player can get though.
3. Do animals that are not in any official book exist in the D&D world at all? How to determine their stats?
Use stats of a similar creature, making adjustments if necessary, or just make them up if they're relevant. Panther stats work for mountain lions lynxs etc. Rabbits can use cat stats with a tweak or two for example. Often Druids may want to know forms in which the combat stats are irrelevant, such as a sparrow for innocuous scouting an suchlike.
Its always the DM's call whether any specific animal exists in the world. Its generally best to assume that every land in the world has a functioning ecology with a lot of normal animals that you would expect to find in a similar habitat on earth. Its generally the more unusual or combat-capable creatures such as Giant versions that will actually require more thought.
I was thinking that the best way to go about this would be to assume only the monsters in the PHB are commonly known to adventurers, whereas all other monsters first need to be encountered in the adventure to be known. And then you learn their abilities by fighting them rather than looking up the MM entry.
"Looking them up in the MM" is how a
player learns a creature's abilities, not how a
character does.
I personally would suggest going on the basis of the terrain that the Druid came from rather than just going by "Everything in the PHB". You can always ask for an Intelligence (Nature) roll to see whether a character might know a specific fact about a particular creature if you're not sure.
Actually, their Int(Nature) skill bonus might not be a bad starting point as to how many 'special' forms a Druid might know well enough to wildshape into.