Fun

Bonnie Invictus, the Goddess of Fun and among the best-kept secrets of the royal family.

Very little is known about the goddess filly who had allegedly died over a millennia ago, or her relationship/status in regards to the rest of her family. Killed before she realised her potential, it is said that the tragedy of the act was what drove GLaDOS in her attempt to commit a mass genocide in her grief. In legends, it is said that Bonnie's murder was literally the reason why adults don't feel Fun as children do, cursed by the alicorn in her last moments.

Wandering as a shade for the past dozen centuries, unable to move on due to her role, she is spurned into resuming a 'corporeal' state by Memories (Atlas). 

At the end of the day, dead is dead, but everything turns out ok in the end.

Portal © Valve
MLP:FiM © Hasbro