Actually they develop in its juvenile state when they are vulnerable to more mundane predators but are then retained for sexual selection in adulthood.

There is only one tarrasque in existence, resulting in it taking its sexual frustration out on level 20 adventuring parties and parties that keep making fun of the lore- uh, history- of the world they grew up in.

It turns out it's man.

The Hobbits just piss them off, then humans have to deal with it..