First: The Mantis Shrimp is beautiful, especially the Peacock Mantis Shrimp:
Second: Their vision is spectacular! They have 16 cones in their eyes. Humans only have 3 - red, green, and blue. Humans can only see derivatives of those three colors because those are the only cones in our eyes. The animal with the next most cones are butterflies - with five. Mantis Shrimp have 16! They can detect ten times more color than humans, including ultraviolet light!
So, they're beautiful and can see an unimaginably beautiful color spectrum.
They are also extremely deadly.
Mantis Shrimp have powerful club-like appendages that fold beneath its body, which makes it look like a preying mantis. These clubs have the ability to strike at the speed of a .22 caliber bullet (50 times faster than a blink of an eye). A blow from these easily strikes through the shell of a crab or mollusk. (There are many videos online of the Mantis Shrimp beating up other animals. So many that I couldn't decide which to post.)
The speed of a blow could also break through glass. Many aquariums do not hold Mantis Shrimp because they will devour other animals in a tank, and because the tank itself won't be able to contain it.
Think about that. Oh, and they are quick swimmers and are very territorial.
Beauty and the Beast, indeed.