UPDATE: Mustangs have been disabled until further notice. The following was posted by Channel Zero:
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.
Source: Official FarmVille Forums