What are companies that have a monopoly in an obscure industry?

There are several 'monopolies' that are not yet discovered or are uncertain.

Toll road and toll bridge operators

In the United States, Toll-booth originated with private investors who could build bridges, and they would charge people, vehicles, and animals to repay themselves. By the 20th century, Toll-Booth control shifted to the state highway departments. Tool-Booths are almost everywhere. The departments collect revenues and price charges on Toll-Booths. For example, the busiest bridge in the world (George Washington Bridge) charges a fee of $14 per car trip, hitting incredible levels of earned revenues.

Games workshop

Games workshop owns more than 80% market share of the tabletop wargaming industry. The game workshop group (GW), are manufacturers of miniature wargames such as Warhammer Age of Sigmar, Warhammer 40,000, and War of the Ring Strategy Battle Game. Recently, the manufacturer became an importer of the US role-playing game (Dungeons and Dragons) and then to wargame and role-playing game publishers in their own right.

Other obscure monopolies include port operators, in other places, controllers of private prisons, canal contractors, etc.