Around here the common name for O. scyllarus is Peacock Mantis Shrimp or Peacock Shrimp. It is a smasher. I'm not sure what the name Clown Mantis refers to.
O. scyllarus is one of the more commonly imported mantis, because they are very colorful and get fairly large. I usually see them for sale between 5 and 6 inches, although I once bought an O. scyllarus that was 7 inches (although it died soon after I got it).
The mantis in my signature below is an O. scyllarus. There is another color morph that is a bright emerald green.
O. scyllarus are fairly easy to keep and are a rewarding animal to have. In my personal experience, they need better water quality than other, smaller types of mantis. As far as care, treat them like a marine fish and you should be in good shape.
If you have more specific questions I know there are several of us here who would be glad to help you out.