Uma Musume: Pretty Derby (ウマ娘 プリティーダービー, Uma Musume Puritī Dābī) is a mobile game by Cygames and still under development.
On April 23rd, 2017, Cygames released a first gameplay teaser. According to it you'll be able to compete in races, train your horse girls, read through story sections and participate in live shows.
In an interview with Famitsu one of the developers of Uma Musume stated that the current development status is around 25%. They plan to release more information about the gameplay at AnimeJapan2017. A short statement on the planned release date was also given: "こ、今年のG1が終わるまでには…..！" ("By the end of this year's G1...!")
You can find the interview here.