A downsized Tarrasque with a few traits removed and replaced.

BOWSER
Size/Type: Large Magical Beast
Hit Dice: 8d10+32 (76 hp)
Initiative: -2
Speed: 20 ft. (4 squares)
Armor Class: 22 (-1 size, -2 Dex, +15 natural), touch 7, flat-footed 20
Base Attack/Grapple: +8/+19
Attack: Bite +15 melee (2d8+7/18-20/×3)
Full Attack: Bite +14 melee (2d8+7/18-20/×3) and 2 horns +9 melee (1d4+3) and 2 claws +9 melee (1d6+3) and tail slap +9 melee (2d6+3)
Space/Reach: 10 ft./10 ft.
Special Attacks: Augmented critical, breath weapon, frightful presence, improved grab, rush
Special Qualities: Carapace, damage reduction 10/magic, fire resistance 10, immunity to poison, disease, energy drain, and ability damage, regeneration 4, scent, spell resistance 12
Saves: Fort +10, Ref +4, Will +4
Abilities: Str 25, Dex 7, Con 19, Int 14, Wis 14, Cha 14
Skills: Listen +16, Search +7, Spot +16, Survival +11 (+13 following tracks)
Feats: Awesome Blow, Improved Bull Rush, Power Attack
Environment: Any
Organization: Solitary
Challenge Rating: 12
Treasure: None
Alignment: Always neutral evil

Bowser stands about 12 feet tall and weighs approximately 1 ton. He speaks Common, Draconic, Abyssal, and Infernal. Bowser's natural weapons are treated as magic weapons for purposes of overcoming damage reduction. The save DC against his Frightful Presence is 16. Bowser does not eat his food raw, so he does not swallow victims whole; he prefers them well-cooked over his lava pits or with his breath weapon. Bowser's Rush speed is 50 feet. Bowser takes lethal damage from acid, cold, electricity, and sonic effects, but is otherwise as difficult to bloody or kill as the Tarrasque.

Breath Weapon (Su): Bowser possesses the same breath weapon as a Young Red Dragon, a cone of fire 40 feet long, which deals 6d10 points of fire damage. The Reflex save DC against Bowser's breath weapon is 18.

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