Hippopotamus (CR 6)

Large Animal
Alignment: Always neutral
Initiative: -1 (Dex); Senses: scent, Listen +7, and Spot +6

AC: 14 (-1 size, -1 Dex, +6 natural), touch 8, flat-footed 14
Hit Dice: 8d8+40 (76 hp)
Fort +11, Ref +5, Will +3
Speed: 20 ft., swim 20 ft.
Space: 10 ft./5 ft.
Base Attack +6; Grapple +17
Attack: Bite +12 melee
Full Attack: Bite +12 melee
Damage: Bite 2d6+10
Special Attacks/Actions: -
Abilities: Str 24, Dex 9, Con 20, Int 2, Wis 12, Cha 3
Special Qualities: Hold breath
Feats: Improved Bull Rush; Improved Overrun; Power Attack
Skills: Listen +7, Spot +6, and Swim +15
Advancement: 9-16 HD (Large); 17-24 HD (Huge)
Climate/Terrain: Warm plains and forests (rivers)
Organization: Solitary, pair, or herd (10-15)
Treasure/Possessions: -

Source: Sandstorm

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

Skills: A 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.

Hippopotami are often brutally territorial and do not hesitate to attack anything they perceive as a threat. Their powerful, tusklike incisors can slice through wood and even thin metal with ease.