The original game that became Monopoly was invented in 1903 under the name The Landlord's Game. It was intended to show how popular economic principles of the time would naturally transfer wealth from tenants to owners, concentrating power in the hands of a few elites and was particularly commonly played for the first three decades or so in economics classes in college. Copycat versions were also released by several companies over the years under names like Monopoly, Fortune and Prosperity. Most notable was the Parker Brother's Fortune, which it designed before entering negotiations with the inventor to purchase the patent in case they weren't able to. (They'd already purchased a patent for a very similar game named Monopoly before finding out about the Landlord's Game, so the purchase was mostly to cover their asses.) But in 1935 they ended up purchasing it for $500 - a little over $9000 today.
Monopoly is now the fifth top selling board game of all time. For context, the top three are chess, checkers and backgammon, which have all existed for hundreds or even thousands of years. Scrabble is the only modern boardgame that outsells Monopoly.