The skeleton is an enemy that spawns in Halloween events since 2012.
Costume
There is also a skeleton costume 
that may be worn by a mouse. It was added with the Halloween 2014 event. Once worn, it will turn the mouse into a skeleton for 20 minutes.
Trivia
- When skills were introduced (V1.82), the players revived with the Ambulance skill had the appearance of a skeleton, but this was later changed in an unknown update.
