Today longhorn cattle are mostly kept as pets. Of course, we don't mean house pets, but they are raised on farms and ranches mostly because it's neat to have longhorn cattle.
Of course, in the United States when you say longhorns you mean Texas longhorns. That is, the cattle that were indigenous to the American southwest. Where these cows came from is a subject of some research and much speculation. But in general they arose mainly from cows that were brought over during the Spanish colonization of Mexico. As the Spanish moved north they brought their cattle and some of their cows got away.
These cows, though, rendered in pastel on hot pressed water color paper, live in central Pennsylvania along with a lot of other critters in a state park. But you can read a bit about their ancestors from the American Southwest - and before - if you click here.