In ancient Greece, there was a bet on athletics, a sport that started the trend. The Romans continued the tradition of sports betting, although their favourite sport was much more macabre: gladiator duels. Both ancient civilizations had fun betting on horse races, mainly chariot races.
Therefore, it is appropriate to say that horse racing became something of a figurehead when sports betting once again gained prominence in society. Thoroughbred horse racing became very popular with the upper class of British society, particularly royalty and the aristocracy. The sport became so popular, with so much money to bet on, that it has become ingrained in British culture. To this day, the British gambling public floods the horse racing markets looking to win a few bucks.
Starting with the horse racing and betting of the British, it seems that sports bets began to be placed on many other popular sports, such as American football, cricket and rugby, mainly about the winners of each game, just like it happened when betting on horse races.