Moonsteed by Manda Benson

"Zeta Verity killed a man who deserved to live, and let a man live who deserved death. Now she has a limited amount of time to stop him and bring justice to Callisto. Born to excel but living in the shadow of her auspicious ancestors, Zeta is posted to the newly terraformed Callisto to work with specially bred horses. Killing a spy in possession of stolen data is just another day's work. She's not impressed by timid and scholarly Vladimir Bolokhovski, who arrives at the research base with a beautiful stallion he has genetically engineered. But when Zeta's superiors start disappearing and an arrest warrant is issued on her for spying, she discovers the man she killed is not what he appeared. Once Verity discovers they're all part of some twisted game, she needs the help of both Vladimir and the dead spy to find and stop the mastermind behind it all."