NCF – Extended list of Neutral creatures.

I believe it was imported from somewhere. You can see a beholder slightly behind it, clearly from the sixth game; a similar story should be true for the beetle as well.

You could say that. Fire Spout was created in an attempt to replicate a spider created by Polish modders, and all parts of its body are model effects layered onto a wolf base. Along with the harpy, this unit requires significant refinement.
The Observer was created using a similar principle. A "body" created from scratch with a texture applied to it, and tentacles that are animated tails of a fire-breathing hound and a spawn of the abyss.
The dragon rider is currently called "Celestial Knight."