This and the following images are based on real-life couple shipping of the real-life horses.
Mares get mated to many different stallions, but often there is one fan-preferred couple. Mejiro McQueen (the racehorse) was said to have a crush for Ikuno Dictus (acted like a shy schoolboy, basically), and from there shipping for the two boomed.
The Umamusume game reflects this by making McQueen and Ikuno roommates and sometimes having McQueen acts as though she has a crush on Ikuno.