Equestrian

The modern circus grew out of horse acts. In 1768 Philip Astley, a skilled equestrian, began giving exhibitions of trick riding in a field in London; in 1770 he hired acrobats, tightrope walkers, jugglers and a clown to fill the pauses between riding acts, and the circus format was born.

So an equestrian living among showmen, acrobats or clowns was a circus rider. In history books the word can also refer to the equites, a class of ancient Rome, which has nothing to do with this trade.