The child was transported by emergency medical personnel to a nearby hospital, according to a release from Yellowstone National Park.
Police said the child was fishing from the shoreline when a beaver came out of the water and charged at him.
The sloth, named Habanero, was part of a group of a dozen sloths given up by Sloth World, a tourist attraction that never opened.
Linh Mai, the first Asian elephant calf born at the Smithsonian's National Zoo in nearly 25 years, is making her debut April 22.
Passengers aboard the Carnival Legend witnessed the dramatic scene unfold during the ship's first night at sea.
Fatou, the world's oldest gorilla living in captivity, celebrated her 69th birthday with a feast Monday, munching on cherry tomatoes, beets, leeks and lettuce at the Berlin Zoo.