Folding Men are a type of abnormals that live all over the world.
They are a possible sub-species of humanity that have gaps in their skeleton that act like hinges allowing them to condense their skeletons, although the procedure causes them great pain. There is a gold-based anti-pain drug, but it is extremely addictive to Folding Men.
In "Folding Man", the Sanctuary team first encountered a folding man when police chased a jewel thief into a basement where he dissappeared. The only way out of the basement was a ten inch wide pipe which they found a finger print in the center of. Also, for some reason, the jewel thief only stole gold and ignored the diamonds and silver. This case reminded Will of a bank robbery he investigated when he worked for the FBI; the robbers locked themselves in the bank vault, but when the police got the vault open there wasn't any trace of them. The only way out of the vault was a nine inch wide ventalation shaft and the only thing the robbers stole was half a million dollars in gold leaving the money behind.
It was discovered by the Sanctuary team that both robberies were committed by a gang of folding men who were being led by a folding man named Nomad. Nomad was making the gold drug and after he got his fellow folding men addicted to it he wouldn't make anymore unless they obeyed him. Nomad's master plan was that after he stole enough gold he'd begin mass producing his drug and get every folding man in the counrty hooked on it. With his army of folding men, Nomad planned to exploit their abilities and create the most powerful criminal organization on Earth.
However, the Sanctuary team was able to stop Nomad and helped all of the folding men that used his drug get clean.