A Mantis Shrimp is commonly a beautiful but deadly animal.  Okay if kept on their own but something you wouldn't want in your reef or fish only tank.  Often brought in on live rock most commonly with Florida Live Rock than any other type.  Here are some photos to help you identify a Mantis Shrimp. Pet Shops often sell these to unknowing hobbyists who see them for their beauty but don't educate them for their razor sharp claws that will cut a fish in two in a heart beat.  This, not to mention that they are extremely difficult to get back out of your tank once introduced. 

Here are a few pictures to help you identify what a mantis shrimp looks like.