There's a couple locations where a snake baddy npc glitches out and respawns a bajillion times before actually spawning for real... you can see it's like maybe 1 out of 50 or 1 out of 100 for them to appear and be the rare version.
The odds for any of them are different though (blobs, bandits, rats, bats, snakes, black blobs, rebels). The chances seem higher for snakes, bats, and bandits, while the others are fairly rare to see.