Just Use Bears, or Wolves, or Spiders, or Tigers

"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. (...) 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."

Just Use Bears, Jack Guignol, 2016

This document takes that idea a bit further. Because yes, a tiger is a bear but stealthier - but isn't a crocodile a tiger except aquatic, if we look at what kind of encounter it'd be? I boiled it down to 12 archetypes that cover nearly any type of creature.

Besides helping with on-the-fly monster encounters, it can also be a tool to help flesh out certain regions, hexes and biomes when worldbuilding/preparing: what would a bull-archetype look like in the desert? A triceratops? A giant scarab beetle?

The methodology and logic is system agnostic - stats are provided for Block, Dodge, Parry or Cairn, and can act as an easy guideline for conversion to other OSR RPGs such as Old School Essentials or Shadowdark.