At last, around the corner poured a huge, slimy leprous gray mass. From its front end rose a pair of hornlike projections, at least ten feet long, with a shorter pair below. The long horns bent this way and that, and Conan saw that they bore eyes on their ends.
Momentarily paralysed with astonishment, Conan stared at the vast mass of rubbery flesh bearing down upon him. The slug emitted a sound like that of a man spitting, but magnified many times over.
Galvanized into action at last, the Cimmerian leaped sideways. As he did so, a jet of liquid flashed through the air, right where he had stood. A tiny droplet struck his shoulder and burned like a coal of fire."
(from "Hall of the Dead", by Howard & deCamp, 1966)
Most of us old grognards understand that original Dungeons & Dragons was designed as a swords and sorcery literature emulator. Modern versions of D&D have lost touch with this fundamental fact, and as Chevski has pointed out, have become self-referential. As I venture into the fantastic fiction referenced in Appendix N of the 1979 AD&D Dungeon Masters Guide, I can't help but grin at passages such as the one above, revealing the roots of so many of the game elements that appear in D&D.
Take the Giant Slug (picture above from Otherworld Miniatures). Like the giant slug in the above Conan tale, the D&D version of the giant slug is gray, with a white underbelly, and spits acid with great accuracy. No one should shy away from using giant slugs in their old school D&D games, knowing that this is a bona fide old school monster.