I love the idea of it. I’m not doing it. For one thing, it adds very little value because my monsters don’t have stat sheets. The characters only see level (at least so far). There would be very little information for the appendix.
The picture that sparked the idea:

If South Dakota can have Jackalope, why not a Creeping Bale Snail?
Wouldn’t it be fun to have an appendix at the end of each book with a Monster Guide in it? Yes, it would be fun. However, it’s also a lot of work not related to the story and if I do it in book one, it will set expectations for the rest. Not happening.
That said, I did pump 32 (8 batches of 4) of these out of Stable Diffusion:
outside, dry hills, photo-realistic, terrifying, gigantic, chupacabra
The ratio of “OK” to “WTF?” is good. Here are the 6 contenders:



That’s the one I’d pick.



Granted there are not that many monster species, yet, but I don’t want to set the expectation. Not to mention that pictures in Kindle books are annoying both to view and to download.