Harpy was a seal at the Langley Falls Aquarium.
When Roger's attempt to find out the cause of Hayley being a grump in "Hayley Smith, Seal Team Six" is interrupted by Stan, she is left mentally as a six year-old.
Jeff and Klaus discover that a news story featuring baby seals being clubbed to death and changed Hayley during her seventh birthday party. Jeff takes her to the aquarium at night with Klaus, hoping that he can trigger a partial regression to her normal self by seeing animals being treated unfairly without killing one, but when Klaus jumps into the seal tank to show how small it is, he finds himself fighting for his life from Harpy, surprised that seals can even climb onto land. With no other option, Jeff is forced to club Harpy to death, but Hayley is distracted by the gift shop and misses it, although two police officers as well as park security cameras capture the event and Jeff is put on trial.
During the trial, Harpy is portrayed as a loving animal and it is a foregone conclusion that Jeff will be found guilty, but Roger's Dr. Penguin hypnotizes the jury into declaring Jeff not guilty instead.