It was Made by Sevelenx!

Garroskull was made by Sevelenx using the Origin Energy of the Perdition Dimension to act as a watchman for it, but also to cause violence for Sevelenx to feed on. Garroskull is one of two Pokemon in the Oracle Titan Duo, and is the smaller one of the two. Many people have speculated the purpose for its wide jaws and lack of hind limbs, and especially how it grew to be as long as it is. DELMAC believes that Garroskull is as large as it is because it's emblematic of the violence caused by Sevelenx, which is significantly larger than Razolteun or Amulteun. Many consider Garroskull a god, and even worship its loyalty. Garroskull was buried alongside Ferramuto, the other Oracle Titan, after the Battle of Order, but has not yet been found.