Looks like a cousin of a mini rabbit snail! Very large. I had one which was very large for a MTS and it was only about 3/4 inch, but noticeably larger than its relatives. Haven't seen it for awhile.
The color on that is interesting it seems very close to the one of mine which was Xlarge. I always wonder where they came from. Almost green grey at the end. I also love the MTS shells which have the lines/ridges. There is quite a variety of colors, patterns and even texture on their shells. They CAN be a nuisance if overfeeding, but I like "MTS" too!