Pato is Spanish for “duck”, as early games used a live duck inside a basket instead of a ball. Accounts of early versions of pato have been written since 1610. The playing field would often stretch the distance between neighboring estancias (ranches). The first team to reach its own casco (ranch house) with the duck would be declared the winner.via Wikipedia
Today, the game involves a ball rather than a duck (the sport was regularly banned in the past due to animal cruelty and violence towards players), and two teams of four on horseback. Each team tries to gain possession of the ball and score by throwing it through a netted ring. Pato shares similarities with polo, in its use of horses to navigate the playing field, and basketball, in its use of a netted ring to score in (although the latter holds its hoop horizontally while the ring in pato is held vertically).
Although pato is Argentina’s national sport, it’s not actually very popular because of its affluent associations. This, from a 2010 Wall Street Journal article*:
Soccer advocates argue that tens of millions of Argentines are fans, with goal posts sprouting up on seemingly every vacant lot and kids booting around bottles or bundled-up-rags if they can’t afford a ball. In contrast, they say, pato enthusiasts number in the thousands, and are relatively affluent and confined to pockets of the countryside. Soccer is “working class [and] inclusive,” while pato is “exclusive and costly,” the Rached bill asserts. In an interview, Sen. Rached adds: “It’s clear that more than 90% of Argentines have never seen a game of pato.”
*I linked to a version of this article from The Wayback Machine to bypass WSJ’s annoying paywall