why doesn't this cow have any horns?

"Mister, why doesn't this cow have any horns?" asked the young lady from a nearby city. The farmer cocked his head for a moment, then began in a patient tone, "Well, ma'am, cattle can do a powerful lot of damage with horns. Sometimes we keep'em trimmed down with a hacksaw. Other times we can fix up the young 'uns by puttin' a couple drops of acid where their horns would grow in, and that stops 'em cold. Still, there are some breeds of cattle that never grow horns. But the reason this cow don't have no horns, ma'am, is 'cause it's a horse."