That's an anti-theft nut that is usually used to assemble bathroom stalls in public places so people can't fiddle with them. They're also commonly used on signs to prevent theft. When installed, they are not intended to be removed. Some idiot put them on the bike to prevent the wheels being stolen so they could lock up the frame and all would still be intact when they returned. Unfortunate for the person that needs to service the bike. Most are simply a soft alloy, IE: aluminum.