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.
Update: A supplementary volume!
I’m going to do that with Marcy’s time training in the dungeon and The Squad’s trip around North Dakota. From the main book’s perspective, it’s all off-screen. However, there is definitely a story there. I’m not sure how long those stories may be because I haven’t written them – and I may not ever write them.
They might make good novellas to publish between books one and two. If I write them before publishing book one, they could be Bonus Scene end-matter. Or, I could package them together with the monster guide a single inter-book novella.
I have no idea if there is going to be something major like that in book two that takes place off-screen or if I will want to write it up even if there is.
Or I could wait until the end of the series and publish The Pack Rising Monster Guide with all of them. I think that might work for a monster guide. It definitely will not work with the cut stories. They need to be closer to where they occur.