What is this bug?

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I came across this bug that I had never seen before. (I thought it was a small dried leaf at first glance.) It moved along like an inchworm. Can you tell me what it is? Is it rare?
—Creeped Out by Critter

This fugly creature looks like a hag moth caterpillar, a.k.a., a monkey slug. It’s covered in dense brown hairs, has nine pairs of fleshy appendages, and uses suction-cup attachments to pull itself around. A monkey slug won’t get much prettier in the adult stage of life: Hag moths have tufts of scales on their legs and an overall shaggy appearance.

These moths are common in North America. But, as you point out, their caterpillars—measuring about an inch (2.5 cm)—are easily mistaken for leaf litter. They probably go unnoticed.

A lot of caterpillars have gnarly-looking spines and barbs, and a handful of species sting. Monkey slugs are one of them—their hollow, quill-like hairs produce a toxin. This usually happens when someone’s hand brushes against one by accident. Dangerous? No. Itchy and painful? Yes.