Dire Hippopotamus (CR 14)

Huge Animal
Alignment: Always neutral
Initiative: -2 (Dex); Senses: scent, Listen +15, and Spot +14

AC: 18 (-2 size, -2 Dex, +12 natural), touch 6, flat-footed 18
Hit Dice: 18d8+126 (207 hp)
Fort +18, Ref +9, Will +8
Speed: 30 ft., swim 30 ft.
Space: 15 ft./10 ft.
Base Attack +13; Grapple +33
Attack: Bite +23 melee
Full Attack: Bite +23 melee
Damage: Bite 2d8+12/19-20
Special Attacks/Actions: Improved grab
Abilities: Str 34, Dex 6, Con 25, Int 2, Wis 14, Cha 8
Special Qualities: Hold breath
Feats: Alertness; Diehard; Endurance; Improved Bull Rush; Improved Critical; Improved Overrun; Power Attack.
Skills: Listen +15, Spot +14, and Swim +20.
Advancement: 19-26 HD (Huge); 27-54 HD (Gargantuan)
Climate/Terrain: Warm plains and forests (rivers)
Organization: Solitary or pair
Treasure/Possessions: None

Source: Sandstorm

Improved Grab (Ex): To use this ability, a dire hippopotamus must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity.

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

Skills: A dire hippopotamus 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.

Dire hippopotami lurk submerged near the shores of their habitats, waiting for prey to come to the water to drink (usually just after dusk and just before dawn). Though not particularly stealthy, they are usually obscured by the water, allowing them to get close enough for a sudden, brutal attack - which is often more than enough.