Its probably a horse conch, here is a picture i found for a small shell. It has the white spot at the end like yours.
Like Sk8r said, these are snail eaters. You can try to feed it with meaty foods and it would probably eat them, but You cant keep it with other snails. Plus they get 2 feet long. If have a sump, you can try to keep it there and release it to the Florida water when it gets too big.