Mask's brindling is not genetic, so it's not from her parents. This type of brindling is a veeeery rare thing that happens when the mare becomes pregnant with twins that later fuse together, merging together into one horse with two sets of DNA. Twins are already extraordinarily rare in horses, and so you can imagine how astronomical the odds are for twins that merge together.
No, it's not genetic, it's purely circumstantial. In horses it's a circumstantial/nonpassable anomaly, whereas brindle in dogs and cattle is a genetically viable/passable coat color.
SO happy to have his grandbaby! :3
he is sooo prettyyyy!!!