So WHY does your father-in-law have this horse if he 1) does not do anything with her 2) does not pay you board for her 3) has not pitched in for any of these fence expenses that this horse is causing.
Even before reading the whole thread, my first suggestion was to sell the horse. Get rid of her. It's not worth your headache or your money. Especially when it's not clear why he even has this half-wild mean horse anyway. She isn't worth much in this market (a mean unbroke horse) but maybe you'd at least be able to give her away. Heck, she may just make a heck of a cow horse, if she's got that much cow in her to chase the cows around on her own.
My mother's horse hates cows. She will also go out of her way to chase them (never through fences though), if they ever get put in with them. And she is one hell of a cow horse. Best for sorting. She'll never let a cow (or calf) get by you.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.