Looks like a Tasmanian Killer Zombie Snail...you'll probably need to sell your house and undergo quarantine and de-contamination procedures, along with the rest of your family and any dogs or cats you might own. You didn't actually put your hands into the water, did you? Good thing you caught this early, otherwise....
Sorry, couldn't resist. It's just a snail (hard to tell from the pic, Ramshorn, I think?), not the AntiChrist. There will almost certainly be more, but they are generally harmless and may be beneficial. They eat leftover food, and depending upon your fish the snails may
be food, especially if you help the process by squishing them against the glass. They tend not to bother healthy plants. They are a normal part of a normal healthy ecosystem.
Please don't fall for the end-of-times panic that many aquarists seem to experience when they see a snail. If they start to overpopulate, that's a good indicator that you are overfeeding the tank. If it gets out of hand and you think you have too many, you can get one or two Assassin Snails (for real, I didn 't make this one up!). The Assassins are very attractive striped snails and in many tanks they do a great job of controlling the population of other snails...emphasis on controlling, not eliminating.
Relax, and enjoy the new addition. Personally, I wouldn't have a tank without them.