The Dalby Spook or Gef was a talking mongoose.
He knew that Harry Price, the Eighth Doctor, Liv Chenka and Helen Sinclair were coming to see him. He had extraterrestrial knowledge which proved to the Doctor that he was not a fake. He broke the Doctor's sonic screwdriver.
The Doctor captured him in a music box, but Helen realised that he wasn't the enemy but Voirrey Irving companion. He was there to protect her from the Sleech. (AUDIO: The Dalby Spook)