The Sundial snail is a common reef aquarium predator that feeds on zoanthid polyps.  These snails are not much bigger than pyramid snails and as you can tell from the picture have unique marking.  As you can see in the picture these snails have a neat little rattle “tail” that resembles one of a rattlesnake. These snails usually come out at night to feed on zoanthid coral polyps but can sometimes be seen when the coral polyps are closed.  As with any new addition to any tank you should fully inspect every piece of coral so that you don’t get unwanted guests such as these snails.  This picture was taken from here.