We came upon this large red slug one evening and because of the hole in its head we at first thought it was a Leopard Slug. After a bit of research, we discovered that it was actually a European Red Slug. The hole is a pneumostome (breathing pore). We were familiar with the Leopard Slug from watching a horrifying video [NSFW] about its mating habits. For a tamer version, you can watch a clip of the original BBC documentary. The documentary is still horrifying, but the commentary is different.