Do you have any plants at all, live ones ? If so, the pond snails almost certainly came with them, quite possibly as eggs, which hatched and are now big enough to see. Eggs can also be laid on hard decor, if you added something from outdoors or even another tank, it could have had eggs on it. It's quite common to have various small snail species hitchhike on plants, but they may also, sometimes, come in the water with fish. If they are very tiny, you might not notice them, but if you dump all the water from a fish bag into the tank, the snail would also be added.
This is one reason to QT new plants, and new fish, in a separate tank, or dip new plants in something that will kill snails. Dipping solutions of Hydrogen Peroxide, or Excel or household Bleach, at the right dilutions, will kill any pest algae and usually pest snails on plants before you add them to a tank.
Always inspect plants before you add them to a tank. Wash in tap water, make sure nothing live is stuck in the roots or under the weight band or in the pot. You can feel egg masses on many of the larger leaf plants, if you run them through your fingers. They'll be small bumps of clear jelly, which are not hard to squish off the leaf.