One of the original monopolies in the US was, IIRC, the Standard Petroleum company. They were good enough to get a significant portion of the market share, but they probably weren't good enough to control the entire oil market. They did so, anyway, by forcing the vibrant (read: highly competitive) Railroad industry to charge extra to other oil companies shipping their products and then sending that excess towards Standard Petroleum, otherwise Standard Petroleum would withdraw their business (which would be a significant to lethal blow to the railroad).
At the time, that was perfectly legal. The thing that makes it illegal was regulation and government intervention.