The ringmaster's outfit was designed to resemble a gentlemen's 18th century riding habit. It is said that ringleader George Claude Lockhart first adopted the look on the orders of British circus owner Bertram Mills, when Lockhart worked for Mills' circus at Olympia, London in 1924. His outfit included "pink" (scarlet) huntsmen tails, a black hat, white shirt and gloves, which has now become the look for the stereotypical ringmaster. A whip is still a standard part of the uniform, leftover from when ringmaster's would lead equestrian acts.
Below are some images of 18th century riding habits, as well as the ringmaster outfit in various forms. I tried to find some images of a man wearing a riding habit, but they are extraordinarily hard to find!
18th century riding habits
Ringmaster George Claude Lockhart and his signature cigar
Ringmaster Charlie Baxter from TV show "7's Circus"
A ringleader costume
Artist's take on the Ringmaster
Bandleader outfits as worn by band My Chemical Romance