The term UPSIDE-DOWN MAMMAL is a crossword solution that consists of three letters: BAT. Bats are the only mammals capable of true flight, and they are also unique in that they are the only mammals that can hang upside down. They have specialized feet and claws that allow them to cling to surfaces, enabling them to hang from ceilings and tree branches. Bats are found all over the world and come in a variety of sizes and shapes.