Let's just say that the artwork is good and creepy from AS&SH.

First some background. I took over a greyhawk 1e D&D campaign from francisca after life started demanding more of his time. The basic idea being a cthulu spin on things. An npc had summoned some help with a war and caught the attention of Dagon. My spin became that Dagon, Cthulu, etc are just avatars/manifestations/echoes of Tharizdun. With an end of arc being the PCs dealing with the original Temple of Elemental Evil. Which they are in at the moment.

They eventually walking through a doorway into a transdimensional kind of area. The caverns had statues of various things: Kuo-toa (deep ones), Mind flayers, demons, devils, black puddings (stole the AS&SH idea they are failed shoggoth summonings), but also Mi-Go and Yiths.

I was going to run the version from AS&SH with tweaks to be compatible with 1e D&D. When they got to the caverns that would lead to a Mi-Go world/place or a Yith place I showed them the illustrations for the Bestiary (since there were statues of them to serve as a warning). I believe their words were, "Winged Nope" for the Mi-Go, and "Conical shaped Nope" for the Yith.

The Mi-Go and the Yith weren't things they had ever encountered so they have no context for them at all. :-)