Lots of creatures within the realm of benchable weight often have specialized defenses that could constitute a 1-hit KO if you fuck up in the slightest.

A chimp could rip your arms off. A kangaroo could crack all your ribs with a kick.

If you consider these “natural weapons” then sure, I guess that just limits you to fighting juvenile cows or something, at which point I’ll give it to you, I could probably take a newborn calf in a fight.