Season 4 Episode 20 - Small Potatoes 

Five bambini in the same town are all born with tails and the local OB-GYN is blamed. However, Mulder discovers the culprit to be a simple man with a genetic deformity who may have the ability to alter his appearance.
parole chiavi: x-files, scully, mulder, full episode, season 4, episode 20, 4x20, small potatoes