Actually, I did a bit of research. Due to something with an H name (Can't remember it for the life of me, bless), some of the hybrids can breed! It is all but unheard of for the colts to have the ability, due to their XY chromosomes being different. Some, although not every, female crosses can. Their XX being the same gives the small chance. They are not usually the most fertile, and it tends to be a bit of a gamble on if their babies will make it. The reason you never see third generation crosses are not see, I believe, is because people don't really want them. They are not guaranteed the novelty of the stripes, and are usually horses with a bad disposition and fewer chromosomes. I thought that I might make her be fertile, but:
1. If it is a colt, it will probably not be fertile. I will be doing RNG with the odds of 75/25 if it will be. If they aren't, they will either be killed off, stay around to help their father control the herd, or just stay in a bachelor band?
2. While females have a higher chance of being fertile, they'll be RNGed as well. 55/45 chances.
3. A lot with her will be left to the RNG gods.
Will the foal be conceived: 50/50
Will the foal develop: 20/80
Will the foal be a live birth: 40/60
I am attempting to make it logical ><