The Vogons were a species of hostile creatures that were once a threat to Earth but were stopped by the Doctor.
The Fourth Doctor told the Hornets that they were not as much of a threat to the planet as the Vogons, Pescatons, Trods or Krynoids. (AUDIO: A Sting in the Tale)
Behind the scenes Edit
The Vogons are a reference to The Hitchhiker's Guide to the Galaxy by Douglas Adams, where they actually successfully destroyed Earth. Adams was the script editor for Season 17 of Doctor Who and also wrote three serials for the show.