A carousel is pretty much the same thing as a merry-go-round; it's a
carnival ride on a large, rotating platform.
Carousel is the proper term if the ride only has horses, whereas
a merry-go-round can have anything (animals, cars, etc.)
The carousel was developed in France where, originally, it was used
to hone lancing skills. A person would sit on the wooden horse
and try to spear a brass ring with his lance.