A month ago on discord, @tomzonia asked this question.

Also wait is the horse is a guy why’s the stupid anime version a girl

I then proceeded to go into autism mode and infodumped a lot about Umamusume.

But I’ll give you the basic answer; there are no male Uma’s in Umamusume, everyone is a female. The way to discover if your horse was a male or a female is by looking at their ear decoration. Left ear means male right means female. So for example, if we take Oguri Cap, who was a male in real life, she has her ear decoration on her left ear.