Zeuglodon (CR 9)

Gargantuan Animal
Alignment: Always neutral
Initiative: +1 (Dex); Senses: blindsight 120 ft., low-light vision, Listen +15*, and Spot +15*

AC: 18 (-4 size, +1 Dex, +11 natural), touch 7, flat-footed 17
Hit Dice: 15d8+135 (202 hp)
Fort +18, Ref +10, Will +11
Speed: Swim 40 ft.
Space: 20 ft./15 ft.
Base Attack +11; Grapple +37
Attack: Tail slam +21 melee
Full Attack: Tail slam +21 melee and bite +16 melee
Damage: Tail slam 4d6+14, bite 4d6+7
Special Attacks/Actions: Stunning strike
Abilities: Str 38, Dex 13, Con 28, Int 2, Wis 10, Cha 6
Special Qualities: hold breath
Feats: Alertness; Diehard; Endurance; Improved Natural Attack (bite, tail slam); Iron Will
Skills: Listen +15*, Spot +15*, and Swim +22
Advancement: 16-30 HD (Gargantuan); 31-45 HD (Colossal)
Climate/Terrain: Cold aquatic
Organization: Solitary or pod (2-3)
Treasure/Possessions: None

Source: Frostburn

Stunning Strike (Ex): Any creature struck with a critical hit by a zeuglodon's tail slam must make a DC 31 Fortitude saving throw or be stunned for 2d4 rounds by the force of the blow. The save DC is Strength-based.

Blindsight (Ex): Zeuglodons can "see" by emitting high-frequency sounds, inaudible to most other creatures, that allow them to locate objects and creatures within 120 feet. A silence spell negates this and forces the creature to rely on its vision, which is approximately as good as a human's.

Hold Breath (Ex): A zeuglodon can hold its breath for a number of rounds equal to 8 x its Constitution score before it risks drowning.

Skills: A zeuglodon has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.

*A zeuglodon has a +4 racial bonus on Spot and Listen checks. These bonuses are lost if its blindsight is negated.

Sailors rightly fear these mighty creatures, for unlike most other whales, zeuglodons are foul-tempered and highly territorial. They are quick to attack anything that intrudes too closely to their territory, and ships avoid waters known to be inhabited by zeuglodons. Although its jaws are filled with massive sharp teeth, a zeuglodon's most dangerous weapon is its tail. These creatures are surprisingly swift and adroit in water, and can slam enemies with their tail with devastating accuracy.