Snails have many predators including: beetles, ants, glowworms, many arachnids - spiders and their relatives, predatory terrestrial snails like glossy snails (Zonitidae), semi-slugs (Testacellidae and Daudebardiidae), decollate snail (Rumina decollata) and others, frogs, toads, snakes, birds,mammals like, red fox, insect eaters, such as the hedgehog, moles and shrews.
To defend against enemies smaller than themselves, snails secrete a foamy mucus, that sticks the predators together and stops them from harming the snail. There isn't anything they can do to defend themselves from larger predators except hide in their shells.
Even people eat snails, usually h. pomatia.
Be careful if you have other pets around your snails, they might think their a tasty snack! I know my cat tries to eat mine all the time!