The amount of times a pet can breed depends on its dominant muscles gene, correct? Well, I've got two unicorns, Aranta and Steel, both with dominant Normal Muscles gene (in fact, both of them have this gene on both sides). But Aranta has 21 total breedings, while Steel has 100. What's more, I bred them together and got this foal, who also has two genes for Normal Muscles, but she has only 15 breedings. If their genetic makeup is identical for the gene that determines breeding, why do they all have different numbers of breedings? Is it a glitch, or are there other factors used to determine number of breedings that aren't publicly available?

It depends on their strength, but you have to research the dominant muscles to see the value. Males get a lot more breedings than females as standard.

I also noticed that when my male leveled, he actually gained more breedings. Most likely because the strength stat increased.

So your foal will likely have way more than 15 breedings if you start to level it.

I think Males automatically get 5 more breedings...