Plesiosaur (CR 10)

Gargantuan Animal
Alignment: Always neutral
Initiative: +4; Senses: low-light vision, scent, Listen +9, and Spot +9

AC: 12 (-4 size, +6 natural), touch 6, flat-footed 12
Hit Dice: 16d8+112 (184 hp)
Fort +19, Ref +10, Will +6
Speed: 10 ft., swim 70 ft.
Space: 20 ft./20 ft.
Base Attack +12; Grapple +36
Attack: Bite +20 melee
Full Attack: Bite +20 melee
Damage: Bite 4d6+18
Special Attacks/Actions: Improved grab
Abilities: Str 34, Dex 10, Con 24, Int 2, Wis 13, Cha 9
Special Qualities: Hold breath
Feats: Alertness; Great Fortitude; Improved Initiative; Improved Natural Attack (bite); Rapid Swimming; Swim-By Attack
Skills: Listen +9, Spot +9, and Swim +27
Advancement: 17-32 HD (Colossal)
Climate/Terrain: Warm aquatic
Organization: Solitary
Treasure/Possessions: None

Source: Stormwrack

Improved Grab (Ex): To use this ability, a plesiosaur must hit an opponent at least two sizes smaller (Large for a Gargantuan plesiosaur) 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.

Hold Breath (Ex): A plesiosaur can hold its breath for a number of rounds equal to 10 x its Constitution score before it risks drowning. For a typical plesiosaur, this is 240 rounds, or 24 minutes.

Skills: A plesiosaur 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.

While not aggressive, a plesiosaur might prod or nudge something that attracts its attention, such as a small boat (which from beneath might rather resemble another plesiosaur) or a clumsy swimmer. If attacked it defends itself vigorously with a powerful bite. A fully roused plesiosaur makes good use of its astonishing swim speed, Swim-By Attack, and the reach granted by its long flexible neck to make swift hit-and-run attacks.