Includes creatures of some sort that live about on the environment without much impacting on gameplay, often having absolutely no meaningful interactions to do with.
Alternate name: Ambient life
The first video game about Ambient creatures was released in 1984.
Commonly small animals like rodents (rats) and small birds (ravens) and other such unobstructive things, including variety of larger (but not gigantic) insects such as roaches.
Killing or otherwise dealing with ambient creatures gives no rewards (no experience, no loot, nothing), and usually no detriments besides announcing your location via your own gunfire or such, except in case of it being a secret.