It may be a leopard or great grey slug. If so, it is native to Europe. It’s supposed to be longer but maybe the cold weather affects its length. If you look closely, you can see a bit of it’s slime trail.
Mollusk: an invertebrate of a large phylum that includes snails, slugs, mussels, and octopuses. They have a soft, unsegmented body and live in aquatic or damp habitats, and most kinds have an external calcareous shell.