Throughout time, people have modified coins by hand-carving or engraving their own designs into existing coins. This form of folk art really took off during the Great Depression, and has been popular ever since. Over time, these 'Hobo Coins' started to incorporate skeletons or skulls, while keeping aspects of the original coins (usually the identifying text and coin date) intact in the final design.
Hobo Coins are not actual US currency or modified US currency coins. They are die-struck new coins at different sizes, weights, and (clearly), different art than actual US coins.