Of course the average answer here revolves around furry porn and/or art directing for sci-fi / fantasy / concept art.
There is no cap to what an independent artist can make. It is all dependent on your own business & promotion skill set. A big part of how independent artists make their money is through:
1.) Commissioned work - Most people understand how this works: Client needs X, artist makes it. Financially successful artists usually avoid personal / private commissions, and instead focus on larger, corporate clients that have much higher budgets. Doing freelance commissions can pay very well for when working for high-level commercial clients.
2.) Licencing - This ties into #1. The more something is used, the more money you should be getting. It's the reason why projects that require the same workload can vary so much in pay from client to client. The same amount of work can generate $400 bucks from a local business, or $8000 from a large corporate client where the work is used more. Work can then continued to be licensed completed for additional use (and additional pay).
3.) Merch sales - This can earn massive bucks, and it's especially useful when it's passive (though passive usually means lower margins). Plenty of artists sell prints, apparel, books, sticker sheets, mugs, etc. with their stuff on it. With decent margins and a modest amount of daily sales, this can really add up. For example, if you're making 16 sales a day with an average margin of $15, that's $240 everyday, $7,200 every month, or $86,400 yearly.