New Yorker

A born-and-bred New Yorker is in the country when he sees a field of animals and says to the farmer, "What a strange looking cow. Why doesn't it have horns?"

"Well, there are several reasons" the farmer replies "Some cows get their horns late, while others have their horns cut off, and still others never even grow horns".

"And this cow?" the city man asks.

"Well, the reason this cow doesn't have any horns is

Spoiler:

that it's a horse".