Druid Wild Shape -- Form Availability

Druid wild shape is a fantastically useful and flexible ability.  An element that I was not properly enforcing is embedded in the first line of that capability:

Wild Shape
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before. You can use this feature twice. You regain expended uses when you finish a short or long rest. (Emphasis added)

Exactly what those beasts are requires a bit of thought.  Typical druids have seen many animals before they start their adventuring career.  Does seeing require 'in person" encounters or would drawings and descriptions suffice?  What animals are likely to have been seen by any particular druid.

Xanathar's Guide to Everything includes a section on learning beast shapes that includes tables that show typical beasts by environment that seems a great starting point.  A druid should be familiar with all of the typical beasts from their childhood environment -- that exist in his/her world.

These tables include dinosaurs, which in my world are very rare at most.  I will typically exclude these 

I am inclined to define the "seen before" requirement to mean that the druid has encountered the animal -or- studied them at some length. 

This sets up the desire for a Druid to learn about many different animal (and perhaps plant, see post on: Moon Druid Wild Shape - Buff at Higher Levels) forms, rather like a Wizard collects spells.

This makes aquiring the weird and exotic forms a lot harder than just finding the appropriate module and I think this is what is intended by the rules.