They 'hitch-hike' into tanks on plants. If you see one, chances are there are more where that came from. They are incredibly tiny as babies, so you could have had them for weeks before they got big enough to be noticeable.
They rarely eat plants or anything and don't get out of control unless you are overfeeding your fish. I have had a handful in my tank for months and find them good for cleaning algae, as well as a good live food for some fish. I would like to keep dwarf puffers in the future soI am letting them stay.
If you don't want them, however, squish it now! And keep squishing all the snails that crop up unless you want them to breed.