"They're Like Cicadas"
[Idly ponders Klyis's suggestion]
[Huh!]
How about like locusts instead?
They are normally just ordinary snails.
Small, harmless, a danger only to the odd lettuce leaf.. and so ubiquitous as to be impossible to eradicate even if you tried.. but every once in a while given the right environmental triggers like the grasshoppers that morph into locusts when seasonal variations in climate are just right to trigger it a generation will develop a different morphology than normal and grow to alarming proportions.
They don't have to swarm like locusts if you don't want them to 🤗
Those hit with this transformation might live a season or years depending on your preference.
They reproduce in both forms, if conditions aren't right giants produce normal snail offspring.
Like locusts there may be decades between occurrences when the climate was just right.
[The environmental trigger for this doesn't have to be climate related .. it could be population crowding, over predation, maybe an abundance of a particular plant or crop .. take your pick]
People may not even realise the two types are the same species.
The environmental trigger doesn't have to be climate.
They may not know what triggers it.