Flail Snail
A monster from the old days of First Edition of Dungeons & Dragons that old school players have fond memories of... and cringe at as well. Like the Flumphs, Vegepygmies, and the Gelatinous Cubes, Flail Snails are ignored for years at a time until some guy at marketing dredges them back up again to generate interest.
Similar to their smaller garden cousins, flail snails are, well, snails. They move really slow, eat plants, and generally, don't pose much of a threat. They are usually found in dungeon and caves feeding on the algae and lichens. Seems simple enough but the snails have means to protect themselves: their flails. These snails have 4 large tentacles that end with a ten-pound mass covered with sharp knobs that deal 1d8 points of damage each, each tentacle having 1 hit die and counting as a separate monster. If all the tentacles are severed the snail dies in 1d3 rounds, emitting a cry each turn that has a 50% chance of having some wandering monster arrive and attack the party.
Aside from their namesake flails, the flail snail also has a brilliantly colored shell that protects the monster with it's 4 AC. The shell itself protects against physical and magic damage, the latter of which has four different effects: 40% of malfunctioning and hitting the closest person, 30% chance of actually hurting it, 20% of doing nothing to it, and a 10% of shooting back at the caster. The shells, however, are very valuable and are used to make 2 shields with the effects for 1d6 months or a robe of scintillating colors.
In the Forgotten Realms, the flail snails were pretty much the same except they ate anything they could fit in their mouths such as vegetation, rocks, sand, and dirt. Their shells could also emit a shimmering light that only made it harder to target their flails.
The snails also had an aquatic variant creatively called the "Sea Snail". Take all of the difficulty of fighting a flail snail, make it 20 feet long, and have it emit a neurotoxin that paralyzes everyone in a 20-foot radius for up to 6 hours. Also, it will summon up to 50 tritons to defend it to the death.
Like all snails, Flail Snails are weak against salt (good luck getting past their mucus) and fear bright lights. They are immune to fire and poison.
Publishing History
Flail Snails, like most of the "goofball" monsters in D&D, debuted in the Fiend Folio for Advanced Dungeons & Dragons 1e. They were updated to 2e in the Greyhawk Adventures Monstrous Compendium Appendix, under the header "Snail", which covered the Sea Snail as well.
In Dungeons & Dragons 3rd Edition, only the Tome of Horrors splatbook by White Wolf, under their Sword & Sorcery subsidiary, covered the Flail Snail. They weren't any better off in Dungeons & Dragons 4th Edition, as their only appearance was in the online April Fools-themed adventure "Fool's Grove", where the party stumbles upon a secret hiding place for an eccentric gnome dedicated to protecting many of the goofier monsters of AD&D, including the flail snail, flumph, carbuncle and umpleby.
Like many of the game's other silly monsters, the flail snail came back in Dungeons & Dragons 5th Edition, in the book Volo's Guide to Monsters.