Limax maximus

Known by the common names great gray snail and leopard snail.
The adult slug measures 10–20 cm (4–8 in) in length. 
The slugs are nocturnal and hide during the day, but can also be seen during the day, especially after rain. As a hiding place, they use heaps of stones made of somewhat coarser pebbles. Because it is damp and cool there and they are safe from their predators (shrews, hedgehogs, etc.).

You can even buy them online. An adult specimen costs up to 18 euros for a slug breeder.