Most animals may not need stats, but that doesn’t mean they may not have a few gameplay-related statistics. Small critters are like living traps: if you fail a check, something bad happens. A small poisonous spider you failed to spot may bite and inject you with a bone-splitting poison (yeah, I don’t know much about poisons,) a swarm of rats may eat your rations while you sleep, or a cat may be startled by your skulking attempts, alerting the whole castle. These may not be your standard encounters, but they add a little variety or, even, realism. Whatever annoyance you can imagine, most of these problems can be presented as ability, proficiency, or skill checks.