Monopolies always form without regulation and it's a natural outcome of pure capitalism.
Then again, there is only one example of a capitalist system in the history of mankind.
All successful (to the society) systems are based on mixed economies (a free market regulated by government).
Give me an example of a monopoly that was not supported by the state. Keep in mind that a Monopoly is not something that innovated and capture market share because they had a better production. So if you are tempted to list something like standard oil you may want to research them first. But ill defend my position, which is not something agent has the ability to do.