Humans are pretty special among mammals. Like all the others, we have hair on our bodies and feed our young breast milk, but we have one anatomical quirk that sets us apart. I’m not talking about our big brains or the walking on two legs thing either. I’m talking about a muscle that sits just under the skin of most mammals - most of them, except us.
The layer of muscle in question is called the panniculus carnosus, which translates to “fleshy cloth” in Latin. True to its name, it’s a thin sheet of muscle attached to the skin and connective tissue across the trunk, back, and upper parts of the limbs in the animals that have it.
The muscle primarily just twitches really fast, which jiggles the skin above it. This is the muscle that lets a horse twitch the skin at the base of its neck, called the “withers”. For example, those twitches help to dislodge insects, birds, or other foreign objects, so the animal doesn’t have to use its limbs or turn its head to get rid of hitchhikers.