Reptilian Gargantua
Colossal Magical Beast
Hit Dice: 40d10+238 (438 hp)
Initiative: +0
Speed: 60 ft.
AC:  32 (-8 size, + 30 natural)
Attacks: 2 Claws +52 melee, Bite +50 melee
Damage: 2 Claws 4d6+20, Bite 4d8+20
Face/Reach: 30 ft. by 60 ft. / 20 ft.
Special Attacks: Trample, Tail Sweep
Special Qualities: Regeneration 10
Saves: Fort +39, Ref +22, Will +13
Abilities: Str 50, Dex 10, Con 44, Int 5, Wis 11, Cha 8
Skills: Swim +30, Climb +30, Jump +30
Feats: Multi Attack, Power Attack, Cleave, Improved Bull Rush
Climate/Terrain: Any
Organization: Solitary (1)
Challenge Rating: ~20
Treasure: None
Alignment: Neutral
Advancement: None

Probably the single most powerful creature of non-magical origins, standing nearly eighty feet in stature, with a length of one hundred and fifty feet from tip of nose to tip of tail. Reptilian Gargantua roam the surface of the planet, alternately hibernating in isolated locales and rampaging across civilized lands. 
These creatures are only vaguely intelligent, but, given their nearly limitless life spans, possess incredible memories, and a keen desire for revenge. Though their rampages are incredibly dangerous, they are predictable due to the incredible amount of noise made by gargantua as the roar and stomp and cut a swath of destruction almost a quarter mile wide everywhere they go. 

Combat
 Nothing a reptilian gargantua does is actually understood by the creature itself, and it fights more as a function of instinct than rage or desire for destruction. It fights with no tactics, and attacks whatever gets close. 
Trample (Ex): A Reptilian Gargantua may trample huge size or smaller creatures for automatic damage. Opponents who do not make attacks of opportunity against the gargantua can attempt reflex saves for half damage (DC 25). Damage is equal to bite damage. 
 Tail Sweep (Ex): The area behind a gargantua, stretching in a half circle with a 90’ radius, is subject to tail attacks. This deals 2d8+30 damage to all who fail a reflex save (DC 25). Those who succeed manage to dodge completely. This attack is obstructed by objects and creatures of size gargantuan or larger, dealing damage only to that object.
Regeneration (Su): All types of damage are regenerated by a reptilian gargantua, including fire and acid. 
If a gargantua loses a limb, it is fully regrown in 1d6 minutes. It is not intelligent enough to place a severed limb to the stump and reattach it.

Special Qualities of Gargantua

Any flower found growing in the footstep of a gargantua can be used in the creation of a potion of enlarge, or potion of bull’s strength, thereby eliminating the materials cost, but not the experience penalty. The flower must grow naturally and not be planted by hand.
A thunderstorm occurs with the birth of a reptilian gargantua. Any dead body struck by one of the bolts is affected as through a True Resurrection spell.