Megafauna, Basilosaurus

This immense primeval whale has a sleek body that looks more serpentine than mammal, and jaws filled with sharp teeth.

Basilosaurus CR 12

XP 19,200
N Gargantuan animal
Init +5; Senses blindsense 60 ft., low-light vision; Perception +25

AC 27, touch 7, flat-footed 26 (+1 Dex, +20 natural, -4 size)
hp 161 (17d8+85)
Fort +17, Ref +11, Will +7

Speed swim 40 ft.
Melee bite +23 (4d6+15/19-20 plus grab), tail slap +18 (2d8+7)
Space 20 ft.; Reach 20 ft.

Special Attacks swallow whole (4d6+15 bludgeoning damage, AC 20, 16 hp)

Str 41, Dex 12, Con 21, Int 2, Wis 11, Cha 6
Base Atk +12; CMB +31; CMD 42
Feats Diehard, Endurance, Great Fortitude, Improved Critical (bite), Improved Initiative, Iron Will, Power Attack, Skill Focus (Perception), Vital Strike
Skills Perception +25, Swim +31; Racial Modifiers +4 Perception
SQ hold breath

Source Wikipedia

Environment any oceans
Organization solitary, pair, or pod (3-10)
Treasure none

Despite their saurian name, basilosauruses are in fact oceanic mammals, and at 60 feet in length are highly formidable beasts of the sea. In appearance, they resemble greatly elongated whales, but with long, toothed jaws resembling an alligator’s. Inexperienced observers often confuse a basilosaurus with a sea serpent or water orm—and in all fairness, in the aftermath of a basilosaurus attack on an unsuspecting ship, such distinctions are largely academic anyway.

A basilosaurus relies as much on vision as echolocation to find prey, but sometimes mistakes smaller ships for food. Basilosauruses have few natural predators; only supernatural monsters like krakens, thalassic behemoths, and the largest of sea serpents can truly threaten a fully grown one.

Section 15: Copyright Notice Pathfinder Roleplaying Game Bestiary 3, © 2011, Paizo Publishing, LLC; Authors Jesse Benner, Jason Bulmahn, Adam Daigle, James Jacobs, Michael Kenway, Rob McCreary, Patrick Renie, Chris Sims, F. Wesley Schneider, James L. Sutter, and Russ Taylor, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.