Monstrous Crab (CR 3)

Large Vermin (Aquatic)
Alignment: Always neutral
Initiative: +2 (Dex); Senses: darkvision 60 ft.

AC: 19 (-1 size, +2 Dex, +8 natural), touch 11, flat-footed 17
Hit Dice: 7d8+35 (66 hp)
Fort +10, Ref +4, Will +2
Speed: 40 ft., swim 30 ft.
Space: 10 ft./10 ft.
Base Attack +5; Grapple +19
Attack: Claw +10 melee
Full Attack: 2 claws +10 melee
Damage: Claw 1d8+9
Special Attacks/Actions: Constrict 1d8+9, improved grab, powerful claws
Abilities: Str 22, Dex 14, Con 21, Int -, Wis 10, Cha 2
Special Qualities: Amphibious, vermin traits
Feats: -
Skills: -
Advancement: 8-10 HD (Large), 11-21 HD (Huge)
Climate/Terrain: Temperate coastal
Organization: Solitary, pair, or swarm (6-10)
Treasure: None

Source: Web

This massive creature scuttles about with surprising speed and grace for its size. Its shell is brightly colored in shades of deep orange to bone white, and its twin claws are both sharp and large enough to behead a horse with one slice.

Monstrous crabs are scavengers, and they feed upon the reeking bodies of dead sea creatures that wash up on shore. Despite this, they aren't above attacking still-living creatures upon sight, since monstrous crabs are always hungry.

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.