So, the characters in your old-school D&D game go somewhere you haven’t yet prepared and you describe some cool, weird-ass monster that you don’t actually have stats for: "At the bottom of the Bone Pit of the Succulent Orb a vast form rises from the sinkhole; its reptilian body glistens with antediluvian slime and its pteroid jaw opens, revealing rows of serrated fangs in what appears to be a most unholy welcome." In situations likes these, I just use the stats for a bear and no one is the wiser. Re-skin appearance, methods of attack, and add special abilities on the fly if you absolutely must...but when in doubt, just use bears.