This little guy was hiding in the shell of one of the Nerites I just added to my tank... the store tank supposedly only had Mysteries and Nerites, although I have a bad feeling that this is a ramshorn. If it’s a mystery though, then I’ll keep it.
If there’s only one and it is rather small (too small to breed) then you will always have just the one. I used to add a single young snail to each tank and because the single rams horn isnt breeding it grew huge, about nickle sized. The red rams are particularly pretty in a dark tank.