Idea:
Change the mare's model so that she can be visually distinguished from a stallion.
Potential implementation:
And no, not in the way you're thinking. For example, by changing her hairstyle. While keeping the stallion's crew cut, give the mares a long mane tucked to one side.
Reason:
You can easily distinguish a bull from a cow, a pig from a boar, a ram from a ewe, and so on with all animals except horses.
This is sometimes necessary and simplifies interactions with animals without having to open a list of livestock or click on each one to determine their gender.