When an ageing racehorse, Makahiki, broke records by winning his first race in five years in Kyoto in early October, Japanese social media was flooded with comments from an unlikely group of racing enthusiasts: gamers.

The online outpouring was the latest sign of boundary-spanning for Uma Musume Pretty Derby, the Japanese mobile gaming industry’s first runaway hit in a decade. The game has found a niche by bridging the historically disparate subcultures of horseracing and women pop stars, known as “idols”...