Here's why it's likely a custom term:
* No Standard Mythology: There isn't a widespread mythology or folklore about creatures called "beaverkin."
* Possible Fan Creation: It's possible someone created the term for a specific setting or purpose, perhaps as a variation on the word "beaver" to create a more fantastical or humanoid creature.
To understand what a "beaverkin" might be, you'd need more context:
* Where did you encounter the term? A game, a book, online forum?
* What is the setting or context? Is it fantasy, science fiction, or something else?
If you provide more information about the source, we might be able to help you understand what the term means in that specific context.