Some gargantuan encounters from the AD&D 2e Monstrous Manual (TSR, 1993), illustrated by Tony DiTerlizzi:
The unique tarrasque (50' long), "the most dreaded monster native to the Prime Material plane," lays dormant for long periods but when awakened it devours all animals and vegetation for miles.
A reptilian gargantua (100-200' tall) is "so immense that it dwarfs virtually all of the world's creatures." Its birth is heralded by thunderstorms that last 101 days, and the lightning from these storms can raise the dead. The flowers that grow in its footsteps can be used to create potions of growth.
A zaratan (200-350' in diameter) is an enormous turtle that spends most of its time sleeping at the surface of the sea, indistinguishable from a small island. An older zaratan may collect a microenvironment of plants and animals on its back, sometimes supporting a village of humanoids or a pirate lair.
