A real thing. Is tolerant of heat up to like 400 celcius (750F), tolerant of heaps of poisonous chemicals that kill other animals and is covered in iron, including its foot.
Also; gastropods have a tendency to shoot darts into one another. These darts have a little sperm package inside, so apparently shooting them into another snail is easier than just handing it to them. Also, Gastropods are usually hermaphroditic, having both sex-bits.
GAMEABLE: It's not a stretch to imagine that maybe if you had a Giant Iron Snail that is immune to fire, it could fertilise its own eggs, put those zygotes into the darts and then shoot darts at whichever animal/human it likes. Then the tiny iron snails grow inside the thing/corpse until they emerge, all irony and fabulous.
Maybe they can also shoot steam . I don't know. Yay.