The Wild Mustang feature has been disabled.
It is not clear whether this is a temporary as glitches are worked out or a permanent disable for the Wild Mustang. If you are already in the process of “wrangling a Mustang” you can still complete it with the help of your friends, but no more new Wild Mustang pop-ups will appear.
From FarmVille Community Manager, Lexilcious:
The Wild Mustang feature has been disabled. If you were already in the process of wrangling a mustang, your Neighbors will still be able to help you complete it. However, no new Wild Mustang pop-ups will appear for users.
Thanks everyone!” (Source: Official FarmVille Forum)