Board Thread:Questions and Answers/@comment-24366751-20170319230341/@comment-24922135-20170320023240

Zs'skyr WAS trapped, but he's the only alien that was.

Sugilite was very poorly explained in that episode and in fact makes zero sense. To try and justify it: Inside every Petrosapien is a small piece of Crystalsapien (Sugilite) DNA. The device Tetrax used to shatter Diamondhead turned that small piece of Sugilite into a full body. Tetrax couldn't shatter himself, he knew Sugilite would need some help from a Petrosapien who knew what was going on. So where would he find another Petrosapien? Ben.

That entire last paragraph was effectively fanfic, but it's the only way I think it would make sense. If Sugilite was never killed by Tetrax I don't think he would ever have been considered 'trapped' inside the Omnitrix.