Well this is weird, when I was drawing this picture I had no Idea there was this "Applejack appreciation day coming up!
And this is Applejack as a filly, so It'll explain the blank flank and seemingly huge hat
But anyways, my theory is that both of AJ, Big Mac & Applebloom's parents are dead. However, I don't think they both died at the same time. I think the father might have been killed in an accident on the farm, and the mother got terribly ill. It's really sad to think about, but I think it also makes Applejack's character more interesting. I also think her hat belonged to her father.
MLP:FiM Belongs to Hasbro and was created by Lauren Faust