Kelpie (CR 10)

Large Fey (Aquatic)
Alignment: Always chaotic evil
Initiative: +5; Senses: low-light vision, scent, Listen +14, and Spot +14
Languages: Common and Sylvan

AC: 22 (-1 size, +1 Dex, +12 natural), touch 10, flat-footed 21
Hit Dice: 10d6+30 (65 hp); DR: 10/iron
Fort +6, Ref +10, Will +10
Speed: 50 ft., swim 50 ft.
Space: 10 ft./5 ft.
Base Attack +5; Grapple +16
Attack: Hoof +11 melee
Full Attack: 2 hooves +11 melee and bite +6 melee
Damage: Hoof 1d6+7, bite 1d8+3
Special Attacks/Actions: Spell-like abilities, trip
Abilities: Str 24, Dex 13, Con 17, Int 10, Wis 13, Cha 14
Special Qualities: Alternate form, cold resistance 20, fire resistance 20
Feats: Alertness; Improved Initiative; Iron Will; Lightning Reflexes
Skills: Bluff +15, Diplomacy +6, Disguise +2 (+4 acting), Hide +5, Intimidate +4, Listen +14, Move Silently +14, Sense Motive +14, Spot +14, and Swim +20
Advancement: 11-15 HD (Large); 16-20 HD (Huge)
Climate/Terrain: Any aquatic and land
Organization: Solitary
Treasure/Possessions: Standard

Source: Fiend Folio

A kelpie usually takes on human form and tries to woo or convince individuals to approach its watery lair. Alternatively, a kelpie remains just under the surface of the water and waits for a victim to get close. It uses charm person to draw victims in and then emotion to either fill victims with fear or root them in place for an easy kill.

Once a victim is close to or in the water, the kelpie reverts to its natural form and attacks. It is adept at tripping opponents in the water so that they are in danger of drowning.

Kelpies can be summoned using a summon nature's ally IX spell.

Spell-like Abilities: At will - detect thoughts; 3 day - charm person, emotion. Caster level 7th; save DC 12 + spell level.

Trip (Ex): A kelpie that hits with a hoof attack can attempt to trip its opponent as a free action without making a touch attack or provoking an attack of opportunity. If the attempt fails, the opponent cannot react to trip the kelpie.

Alternate Form (Su): A kelpie can assume the form of a human (either gender) as a standard action. Switching back to its natural form is also a standard action.

Scent (Ex): A kelpie can detect approaching enemies, sniff out hidden foes, and track by sense of smell.

Skills: Kelpies have a +8 racial bonus on any Swim check to perform some special action or avoid a hazard and can always choose to take 10 on Swim checks, even if distracted or endangered.

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.