It actually spawned in his "Dungeon Guardian" form when I used the spawner. It becomes a pain if you do a new character and spawn it, having a chance to do that. Good thing I was in a party with all my friends with overpowered characters.Have you beat him yet? Your post was a while ago. The spinning head mode is one of his moves. How long did it take to go into spinning mode. Did it actually spawn in that mode? Here's a link for the wiki: http://terraria.wiki.gg/Skeletron_Prime