Two cows are chatting out in the field. Cow One says, "So, whad'ya think about this mad cow disease thing that's goin' around?"
Cow Two says, "Oh, it doesn't really worry me. But then, I'm a horse!"
You see, he wasn't really a horse. See?
That's why it's funny. He really was a cow. Not a horse.
The fact that they are talking makes it funny, too. Cows don't talk. At least, not so we can understand them. They moo. Like, "Mooooo."
So if they can't really talk, it's funny to say "Two cows are chatting out in the field." See?