The Fontana dei Quattro Fiumi ("Fountain of the Four Rivers") represents one river from each of the four known continents at the time. Each river is surrounded by a few sculptures of plants and animals found on that continent.
The only continent Bernini had never personally been to was America (North & South were considered one continent), so he had a friend describe an Armadillo to him. This was how Bernini interpreted the description. Awesome!