Acidborn Shark, Large* (CR 3)

Large Magical Beast (Aquatic and Augmented Animal)
Alignment: Always neutral
Initiative: +6 (+2 Dex, Improved Initiative); Senses: blindsense 30 ft., keen scent 180 ft., Listen +8, and Spot +7

AC: 17 (-1 size, +2 Dex, +6 natural), touch 11, flat-footed 15
Hit Dice: 7d10 (52 hp)
Fort +10, Ref +7, Will +3
Speed: Swim 60 ft.
Space: 10 ft./5 ft.
Base Attack +5; Grapple +12
Attack: bite +7 melee
Damage: Bite 1d8+4 plus 1d6 acid
Special Attacks/Actions:
Abilities: Str 17, Dex 15, Con 17, Int 1, Wis 12, Cha 2
Special Qualities: breath acid, Immune to acid, poison
Feats: Alertness, Great Fortitude, Improved Initiative
Skills: Listen +8, Spot +7, and Swim +11
Advancement: -
Climate/Terrain: Any dungeon
Organization: Solitary
Treasure/Possessions: None

Source: Dungeonscape

Breathe Acid (Ex): An acidborn shark is capable of breathing acid just as it once breathed water. It is in no danger of drowning if immersed in acid. However, the shark cannot breathe in regular water, though it can hold its breath for as long as it is able.

Keen Scent (Ex): A shark can detect creatures by scent in a 180-foot radius and detect blood in a 1-mile radius.

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.