If you run Druids by-the-book, then they can only Wild Shape into a limited number of forms that the Druid has seen:

DMs have a lot of control over Wild Shape. Since the Druid doesn’t get Wild Shape until Level 2, you aren’t committed to allowing the player a certain number of Wild Shape options in their backstory. Rather, you can cater their options to the campaign world by exposing the party to certain creatures as they progress. I recommend keeping this within reason, as the character is likely to have seen a certain number of beasts by the time they become an adventurer.

If you lean on your characters to develop adventures (as I highly recommend), you can get a little more inspired with this. For example, you might require the Druid to fully study and observe the specific beast they want to mimic. For inspiration, I recommend adapting what Dael Kingsmill suggests for locating animal companions.

Whether you’re a DM selecting what beast options to make available to your Druid, or you’re a player selecting them as an aspect of their backstory, it can be a pain for Druids to track what options they have available as Wild Shapes. Especially so if they’re allowed to change into anything within the CR limitations. Fret no more.

I’ve created a checklist to help Druids keep track of their Wild Shape options:

Click the image to pick up the supplement on the DM’s Guild. It includes a list sorted by power, with the following statistical summaries:

Challenge Rating (CR)

Size

Defensive Stats (AC/HP)

Offensive Stats (Attack/Damage)

Movement (Base/Climb/Swim/Burrow/Fly)

Perception (blindsight/darkvision, advantage on sight/hear/smell checks)

Since the beasts available scale differently between Circle of the Moon Druids and their brethren, I’ve labeled creatures which have a swim or fly speed with an asterisk, so you know they might still be level-restricted when considering your options.