Horse racing is often called the “sport of kings,” a term cited in print since at least the 1700s. The game of polo has been called the “sports of kings” since at least 1896, when the description appeared in an “Astor Cup” polo article in the New-York (NY) Daily Tribune.
 
 
Wikipedia: Polo
Polo (Persian: چوگان‎, chowgan) is a team sport played on horseback in which the objective is to score goals against an opposing team. Sometimes called, “The Sport of Kings”, it was highly popularised by the British. Players score by driving a small white plastic or wooden ball into the opposing team’s goal using a long-handled mallet. The traditional sport of polo is played at speed on a large grass field up to 300 yards long by 160 yards wide, and each polo team consists of four riders and their mounts. Field polo is played with a solid plastic ball, which has replaced the wooden ball in much of the sport. In arena polo, only three players are required per team and the game usually involves more maneuvering and shorter plays at lower speeds due to space limitations of the arena. Arena polo is played with a small air-filled ball, similar to a small soccer ball. The modern game lasts roughly two hours and is divided into periods called chukkers. Polo is played professionally in 16 countries. It was formerly, but is not currently, an Olympic sport.
 
(Oxford English Dictionary)
sport of kings n. (with the)  (a) hunting;  (b) warfare;  (c) horse racing (now the usual sense);  (d) surfing.
[a1668   W. Davenant Wks. (1673) i. 322   For I must go where lazy Peace, Will hide her drouzy head; And, for the sport of Kings, encrease The number of the Dead.
1702 E. Ward Bribery & Simony 11   War is the Sport of Kings and mighty Lords.]
1735 W. Somerville Chacei. 14   My hoarse-sounding Horn Invites thee to the Chace, the Sport of Kings, Image of War, without its Guilt.
1744 Review 12   Ths War to Britain sure Destruction brings, War Bane of Subjects, and the Sport of Kings.
1843 R. S. Surtees Handley Cross I. xiii. 253   ‘Unting is the sport of kings, the image of war without its guilt, and only five-and-twenty per cent. of its danger!
1859 H. H. Dixon Silk & Scarlet 85   The names of ‘Bolton’, ‘Queensberry’, and ‘Rockingham’ had, it is true, lent lustre to ‘the sport of kings’.
1886 Times 27 Mar. 4/1   The decay of hunting would be little short of a national calamity. The sport of kings exercises a healthy influence upon the national character, because it affords an opportunity of keen enjoyment to both gentle and simple.
1918 G. Frankau Poet. Wks. (1923) II. xxi. 130   Weep for the King of Sports, the Sport of Kings;‥On thousand tracks, unridden, desolate, Hay waves from winning-post to starting-gate.
     
15 September 1896, New-York (NY) Daily Tribune, pg. 12, col. 2: 
THE CHAMPIONS AT POLO.
(...)
It showed the quality of the game, and the intense interest the crowd takes in the “sport of kings.”
 
Google Books
Behind the Screens;
An English woman’s impressions of Japan
By Evelyn Adam
New York, NY: G.P. Putnam’s Sons
1910
Pg. 21:
Polo, the sport of kings, is entirely out of fashion.
 
Google News Archive
8 June 1913, The Gazette Times (Pittsburgh, PA), “Local Men Play at Sport of Kings in Greensburg and Own Blooded Ponies” by M’Cready Huston, pg. 1, col. 3:
GREENSBURG, PA, June 7.—An impetus which may lead to a revival of the game is being given polo, the sport of kings, by several Greensburg and Pittsburgh men.
 
29 June 1913, Chicago (IL) Daily Tribune, “News of the Society World” by Mme X, pg. H3:
CHICAGO’S gay and fashionable world has been quite absorbed in polo at Lake Forest this last week. This is justly called the sport of kings. It is no game for a poor man, no matter how good a horseman he may be.
 
OCLC WorldCat record
Polo Was Sport of Kings, Veiled Ladies Played, Too
Edition/Format:  Article : English
Publication: The Science News-Letter, Feb. 22, 1936, vol. 29, no. 776, p. 115
Database: JSTOR
 
OCLC WorldCat record
POLO EXPRESS With more patrons and money being attracted to this country, Yolanda Carslaw explores how the sport of kings is growing
Publisher: [London : S.n, 1901-
Edition/Format:  Article : English
Publication: Country life. 201, no. 28, (July 12, 2007): 70-73
Database: ArticleFirst
 
CBS News
April 8, 2012 7:57 PM
The Sport of Kings: Polo
The following script is from “The Sport of Kings” which originally aired on April 8, 2012. Lara Logan is the correspondent. Max McClellan, producer.
 
Polo is called the sport of kings and for centuries its drawn royalty to its ranks. In the minds of many, the game has always belonged to the rich, the famous, and the privileged few. But there’s more to the sport than the glamorous world that surrounds it.
(...)
Many people describe polo as hockey on horseback. Nacho told us it’s more like playing golf in an earthquake.