Providing Donald Trump’s $175 million appeal bond when other insurers wouldn’t is business as usual for California financier Don Hankey. As chairman of the Los Angeles-based Hankey Group of Companies, which includes an insurer, a subprime auto lender and a commercial real estate investment firm, Hankey has amassed a fortune lending to borrowers other financial firms shun. Hankey’s assistance to Trump has brought the little-known billionaire into the spotlight. But in recent years, several of his companies’ operations attracted the attention of the U.S. Justice Department, the Consumer Financial Protection Bureau and the California Department of Insurance. Since 2015, regulators...