Most umamusume are based on stallions because most RL racehorses are stallions.
This is also hinted in their character designs, in particular the positioning of their eardecs/hairdecs/headwear. Stallions have them on the right (or weighted to the right), while mares get them on the left. Golshi has a purple bow near her right ear, and Rice Shower wears her hat off-center near her right ear (and tilted downwards to the right).
The only exceptions are certain anime "umas with their names changed because they can't secure the rights to the name" who get no hairdecs/eardecs/headwear (though their hair have very slight asymmetry). And Fine Motion, who has completely symmetrical hair decs and whose real-life counterpart is (widely rumored to be) a futa.
