Hippocampus (CR 2)

Large Magical Beast (Aquatic)
Alignment: Always neutral
Initiative: +2 (Dex); Senses: darkvision 60 ft., Listen +6, and Spot +6

AC: 14 (-1 size, +2 Dex, +3 natural), touch 11, flat-footed 12
Hit Dice: 3d10+9 (25 hp)
Fort +6, Ref +5, Will +4
Speed: Swim 50 ft.
Space: 10 ft./5 ft.
Base Attack +3; Grapple +10
Attack: Tail slap +5 melee
Full Attack: Tail slap +5 melee and bite +0 melee
Damage: Tail slap 1d6+3, bite 1d4+1
Special Attacks/Actions: -
Abilities: Str 16, Dex 15, Con 16, Int 5, Wis 13, Cha 8
Special Qualities:
Feats: Alertness; Iron Will
Skills: Jump +11, Listen +6, Spot +6, and Swim +11
Advancement: 4-6 HD (Large); 7-9 HD (Huge)
Climate/Terrain: Temperate aquatic
Organization: Solitary, pair, or pod (5-12)
Treasure/Possessions: None

Source: Stormwrack

A hippocampus prefers to outrun danger. Hippocampi are free spirits, making it difficult to teach them the discipline required for combat mounts. For those who succeed, a battle-trained hippocampus lashes with its powerful tail and can bite its rider's opponents.

Skills: A hippocampus 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.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.