He's not a mystery, that's just what they're called. Look it up. :) (They're also called apple snails.)
I just got this little guy today, to help clear the tank of algae. Unfortunately, it has become a problem. :(
The last time I had one of these, he grew to darn near fist size! This one is about 1 1/2 inches in length, so far. My daughter picked out a yellow one, but they also come in black. Yellow is easier to see against the black background of the tank.
Yes, I know we're back to my fish tank, I just didn't do anything of interest today, so this is what you all get. Sorry. :)