Mosasaur (CR 10)

Huge Animal
Alignment: Always neutral
Initiative: +1 (Dex); Senses: low-light vision, scent, Listen +12, and Spot +12

AC: 19 (-2 size, +1 Dex, +10 natural), touch 9, flat-footed 18
Hit Dice: 15d8+99 (166 hp)
Fort +17, Ref +10, Will +6
Speed: 10 ft., swim 50 ft.
Space: 10 ft./5 ft.
Base Attack +11; Grapple +28
Attack: Bite +18 melee
Full Attack: Bite +18 melee and tail slap +13 melee
Damage: Bite 2d8+9, tail slap 1d8+4
Special Attacks/Actions: Improved grab, swallow whole
Abilities: Str 28, Dex 13, Con 22, Int 2, Wis 13, Cha 9
Special Qualities: Hold breath
Feats: Alertness; Great Fortitude; Improved Natural Attack (bite); Toughness (x3)
Skills: Listen +12, Spot +12, and Swim +17
Advancement: 16-29 HD (Huge); 30-45 HD (Gargantuan)
Climate/Terrain: Warm aquatic
Organization: Solitary or pair
Treasure/Possessions: None

Source: Stormwrack

p>Improved Grab (Ex): To use this ability, a mosasaur must hit an opponent at least two sizes smaller (Medium for a Huge mosasaur) with its bite attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the target in the following round.

Swallow Whole (Ex): A mosasaur can try to swallow a grabbed opponent of up to two sizes smaller by making a successful grapple check. Once inside, the opponent each round takes 1d6+9 points of bludgeoning damage plus 1d8 points of acid damage from the mosasaur's digestive juices. A swallowed creature can cut its way out using a light slashing or piercing weapon by dealing 20 points of damage to the mosasaur's digestive tract (AC 13). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.

A Huge mosasaur's gullet can hold 2 Medium, 8 Small, 32 Tiny, 128 Diminutive, or 512 Fine or smaller opponents.

Hold Breath (Ex): A mosasaur can hold its breath for a number of rounds equal to 6 x its Constitution score before it risks drowning. For a typical mosasaur, this is 132 rounds, or over 13 minutes.

Skills: A mosasaur has a +8 racial bonus on any Swim check to perform a 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 mosasaur attacks with its huge, gaping jaws, and can deliver powerful slaps of its crocodilian tail as well.