Not quite cowgirls but...

Lionesses hold down a boar while they chew his balls and cock off, while he's still alive and struggling:
http://www.youtube.com/watch?v=e5OweMjFrIs

Female hyena grabs wildebeast bull by the balls and hangs on while he runs until she pulls his ballsack off:
http://www.youtube.com/watch?v=0KBbN5z5nTE