Birds such as thrushes and blackbirds will eat slugs. Hedgehogs are big slug and snail eaters. Even badgers and foxes have been known to eat slugs although both are said to prefer earthworms given the choice. One of the most voracious slug eaters known to gardeners is the duck. They greedily shovel up any slugs they can find. This might be a little unfortunate as not all slugs eat plants. Many survive on decaying plant matter and so complement to a healthy garden rather than a threaten it. Toads are supposedly great at scoffing slugs. You can encourage them to love in your garden by leaving some upturned terracotta plant pots lying around. Toads need peaceful, damp, dark places like the underside and rear of sheds top be happy in your garden. Hens are also excellent slug eaters but they come with a downside. A happy foraging hen will think you have planted all your seedlings just for her and do more damage in an hour than a slug will do all night.
Some of the well known things that eat slugs are birds, badgers and foxes but is they had a choice to pick an earth worm over a slug they would pick the earthworm. Hedgehogs, ducks toads, and hens.