There’s a snail you can encounter in Dungeons & Dragons. It’s called a Flail Snail. It’s called that, because it has organic, spiky flails where it’s eye stalks should be. It definitely exists purely because ‘snail’ and ‘flail’ rhyme.
At least the snail from Fantastic Beasts is a significantly different monster. Not like the hippogriffs and griffins, where I was having to split hairs to make a monster that was distinctly different from what was there before. The Streeler is a very different kind of strange snail.
Streeler

When you’re throwing a monster like this into your adventure, the important thing to remember is that a slow movement speed can make the Challenge Rating mismatched. If the party has range attacks and keeps moving, they could avoid contact with it really easily, slowly chipping away at it.
Of course, killing this thing may not be the mission. The trail is a problem, but the party might be tasked with steering a swarm of Streeler away from farms and villages, but they have to get creative with how they move it and keep it alive.
The Streeler is certainly not going to be hard to find on an adventure, the corroded groove it leaves in the world will be easy to spot, though stepping into it will likely be the party’s first mistake.
I must admit, when I read ‘giant snail’, I pictured a gargantuan monster. But wizards have kept Streeler for their own personal studies, and that’s hard to accomplish if the Streeler is the size of a house. I still went with ‘medium’ size, which is probably too big, but if don’t give it some strengths it’ll feel embarrassed in front of the flail snail…
Thank You For Reading
The rest of the Fantastic Beasts have D&D stats! You can find them here!