They are indeed one of the ramshorn snails. They usually arrive in the tank on live plants either as the snail or eggs.
I have had these snails in my tanks for years. Once upon a time I tried to get rid of them and failed so now I just leave them there. Mine never grow more than a couple of mm across. Over feeding the fish is the main cause of too mnay of them, but a few are a useful part of the tank eco-system.