Whenever this pops up, I keep wondering if there's any precedent for antitrust rulings against companies in industries where they don't exactly hold a monopoly,

Whenever this pops up, I keep wondering if there's any precedent for antitrust rulings against companies in industries where they don't exactly hold a monopoly, but have essentially created their own market-like structure while also competing in that market themselves. Do comparable concepts even exist in other industries?