The only appendage that hangs from the bottom of a male horse that even remotely resembles a cow's teat, is a penis. This is exactly where this joke always went, and I heard it over 50 years ago. As soon as Laura said it was a male horse the joke was about masturbating the horse, it had nothing to do with trying to milk a mare.
What's your favorite joke?
To you.
Tell me, when you see two oranges in a bowl, do you immediately think of breasts?
Its about grabbing a horse penis thinking its an udder, not some sick sex thing.
Its a shame your mind goes there. I grew up on a farm. We knew that thing was there and we knew what it was used for, but no one was stupid enough to try to grab it.. gees... get your a$$ kicked. Not without a pinch pen and a couple of beefy guys to help, but thats a different scene completely. And a horse or a bull will do it too. Kick your tail and make you completely sorry you ever got near there. But, thats what makes the joke funny.