If you have had a Wild Mustang show up on your farm, or were waiting on one, the wait is a little longer as Zynga has disabled this item. They had problems with the email function of the Wild Mustang, but hopefully it will return shortly. Here is their official statement.
Due to some unforeseen technical difficulties (bad URL in the Mustang email, pop-up for the Mustang not appearing but displaying the progress icon, etc.) the Mustang has been disabled for the time being. Once these errors are corrected, the Wild Mustang will be enabled once more. We apologize for the inconvenience, and will update you via this thread as soon as we have more information.