Bats are the only mammals that truly fly, but they aren't the only ones you might see swooping overhead at dusk. For tens of millions of years, a variety of other furry vertebrates have also been soaring through forests, especially after dark.
Flying squirrels — which actually glide, not fly — date back to at least the Oligocene Epoch, and now come in 43 species across Asia, Europe and North America. They sail from tree to tree on a special membrane between each front and back limb, a trick that has evolved multiple times in history. (Aside from flying squirrels, it's also used by other aerial mammals such as anomalures, colugos and sugar gliders.)