Zaratan (CR 20)

Colossal Magical Beast
Alignment: Usually neutral
Initiative: +0; Senses: darkvision 60 ft., low-light vision, Listen +12, and Spot +12
Languages: can understand rudimentary commands in most languages, including Common, but speak only Aquan

AC: 32 (-8 size, +30 natural), touch 2, flat-footed 32
Hit Dice: 38d10+345 (554 hp)
Fort +30, Ref +21, Will +16
Speed: 10 ft., swim 30 ft.
Space: 100 ft./40 ft.
Base Attack +38; Grapple +51
Attack: Bite +46 melee
Full Attack: Bite +46 melee
Damage: Bite 4d6+24
Special Attacks/Actions: Fin sweep
Abilities: Str 43, Dex 11, Con 28, Int 6, Wis 19, Cha 12
Special Qualities:
Feats: Alertness; Toughness
Skills: Listen +12, Spot +12, and Swim +24
Advancement: 39-76 HD (Colossal)
Climate/Terrain: Warm aquatic
Organization: Solitary
Treasure/Possessions: Double standard

Source: Arms and Equipment Guide

Fin Sweep (Ex): The zaratan thrashes all four of its fins simultaneously, affecting everything in the water in a 40-foot radius. Objects and creatures within the swept area automatically take 2d8+16 points of bludgeoning damage. Affected creatures can attempt a Reflex save (DC 38) to take half damage.

If attacked, zaratans prefer to flee rather than fight, but they are able defenders. At first sign of aggression, a zaratan pulls its head and flippers into its shell, waiting for the attacks to stop. If the attacks continue for more than 2 rounds, it lashes out with its bite and sweeps with its fins for 1 round, then swims away in the next round. It continues to alternate attacking and fleeing until the opponent is discouraged or dead. If a zaratan knows it has riders, it will not dive underwater unless its life is in danger.