Dheiju's Snail

While the overall species is one that does exist, they are a rather typical species of snail with typically gray or green shells that don't do well with salt or with speed. It can be fairly difficult to differentiate these specific snails from others like them, in fact.
Usually, when someone is talking about Dheiju's snails, they're referring to the specific mythological snail who is attributed to the Low god Dheiju, meant to oversee crops. And indeed, unlike other snails, these particular ones are good for growing things, consuming the sickly plants with avarice and leaving the good ones alone, and a healthier crop overall.
They are usually found on the stalks of plants, even when not eating. They can get quite far up, and their very sticky slime allows them to stay relatively stable, even in high winds.
Discarded shells make common souvenirs for the region.
When it comes to locating the correct plant, they can be quite determined in how they get there. It's unclear how they determine that a plant is theirs for eating, but once they have located this plant, you can move them a hundred meters away and they will eventually find their way back to that spot.
Their status as a mythological figure comes from their association with Dheiju, god of crops. The snail is his favored animal companion, and while he is occasionally mocked for this choice, it always goes badly for the mockers. The snail has little to say, but is usually credited with wise words when it does speak.