In order to unlock this achievement you need to collect a total of 50 Mystery Foxes.Beware that some of the Mystery Foxes need to be found while in the Story Chapters.
The remaining are found in Free Ride. You can access it from the Main Menu after playing the first Story Chapter.To find most of them look for a white fox icon on a wall or fence. This indicates that the Mystery Fox is nearby.
Video with all Mystery Foxes: