What I've come to find an interesting element of monopolies is that virtually all that I can think of seem to be fundamentally
networked entities. That might be physical (railroads, utilities, communications, broadcast) or logical (marketing, wholesale, retail), and might involve edges/links (rail, comms), vertices/nodes (ports, datacentres), or quite often a mix of both.
All tend to see some large realm over which there is an increasing return to scale (if there weren't, monopolies wouldn't form), and quite often a single entity may be established over multiple networks or modalities (e.g, Google controlling search, online advertising, and Web browser development).
I've been hard pressed to come up with examples of monopolies which aren't describable as networks, though that description may not at first be obvious.
Non-government-sanctioned monopolies exist as (illegal) drugs cartels, criminal syndicates, and warlord hierarchies, in which growth and maintenance of the network is typically obtained through direct (and non-legally sanctioned) force of arms, coercion, or intimidation.