Skeletron X

Skeletron X is a boss that has 8 arms and a skull 1.5x bigger than Skeletron. Like Skeletron Prime, he is made of metal. He takes 5 times as many hits to kill as Skeletron Prime. When you defeat him you get 2 platinum coins, and an item that summons a pet skull (it looks like a mini version of Skeleton X and can fight)