I'm a little late today, whoops!
There are some creatures in the game that don't seem suited to combat encounters at all, but instead are designed for roleplaying encounters, such as the Sphinx, or for scenes that evoke wonder. The
Flail Snail is, I think, one of the latter.
The above image depicts one of the mounts in the Neverwinter MMO. It isn't the worst one in that game - a title which surely goes to the Giant Crab, atop which the rider just sort of awkwardly stands, like a colossal fool. The picture in Volo's is really nice, with beautiful colours and a real sense of stickiness about the whole thing. It makes for an image that both attracts and repels simultaneously, which I think is pretty interesting.
These critters are large elementals, presumably meaning that they are native to the Plane of Earth, but the appendix indicates that you can find them in Swamps, Forests and Underground, so they presumably wander through portals and then sort of spread out. A day in the life of a Flail Snail apparently consists of oozing along, eating everything on the ground that it passes over, with a preference for gem and crystal deposits. As they do so, they leave behind a trail that can be turned into glass windows and objects, which is pretty fun to incorporate as a world-building element. It's as dumb as, well, a giant snail, and it cannot speak, so we're not to imagine some grand culture here.
The Flail Snail's shell can be harvested (upon death, I think it is safe to say), and it is useful for creating
Spellguard Shields, shields that in general have
anti-magic properties, and
Robes of Scintillating Colours. So I guess that they must be hunted down a lot, given that the going price for the shell is 5,000gp; I imagine that you'd get conflict between such hunters, and whoever is harvesting the glass trail, which is one potential use for these guys as a plot hook.
The statblock for this guy is really impressively huge, being more complex than any of the NPC ones in the back, and longer than the CR 10 Froghemoth opposite. It is CR 3, and its defences seem pretty standard for that, although it is truly slow at 10ft. It has five flails, and can attack with them of them on an attack action, reaching out to ten feet for 6 damage per hit. Somewhat complex is the rules for taking damage, wherein the Snail loses a Flail whenever 10 points or more of damage is taken on a turn. This, combined with the
Antimagic Shell, means that you'll be more pushed to read this on the players' turns, rather than on your own. This latter property is fun, and can reflect spells back at casters, or make big bangs, as well as generally giving disadvantage to cast the spells in general. The Flail Snail finishes this heady array of powers with
Scintillating Shell, allowing it to start a private disco that can Stun attackers, and will give them disadvantage to attack.
Now, I think that having one of these just sitting in a cave, waiting for your players to kill it, is going to be tragically misusing them. I think that far more benefit can come to your campaign if the players find themselves driven to
defend the Flail Snails. Part of the reason for me saying that is their death throes, in which they retract into the shell and begin wailing for several minutes, being heard out to 600ft; they can be healed by the
Regenerate spell to stop the process. This honestly sounds like a way to either make your players feel bad, for killing something that then
cries piteously as it dies, or as a way to draw them in to find poachers killing off some of these magnificent critters. Picture the scene: as they wander through the swamp, they can hear one of these dying in the distance, and maybe even arrive in time to heal it and drive off whatever attacked it. That leads into a whole segment - who is killing the Flail Snails? How do the party feel about that concept? Do the hunters have useful information that the party can use, and will they overlook this activity in order to get it? Does hunting Flail Snails differ from hunting Cows?
Rangers and Druids can be tough to give personalised stories to - 'protect nature' seems more of a downtime goal than an adventuring one - but this might be the perfect way to let them do just that in game, while also generating a way for players to explore how their characters feel about nature and their place in it.
These things are still mad as balls, though.