Porpoise (CR 1/2)

Medium Animal
Alignment: Always neutral
Initiative: +3 (Dex); Senses: blindsight 120 ft., low-light vision, Listen +8*, and Spot +7*

AC: 15 (+3 Dex, +2 natural), touch 13, flat-footed 12
Hit Dice: 2d8+2 (11 hp)
Fort +4, Ref +6, Will +1
Speed: Swim 80 ft.
Space: 5 ft./5 ft.
Base Attack +1; Grapple +1
Attack: Slam +4
Full Attack: Slam +4 melee
Damage: Slam 2d4
Special Attacks/Actions: -
Abilities: Str 11, Dex 17, Con 13, Int 2, Wis 12, Cha 6
Special Qualities: hold breath
Feats: Weapon Finesse
Skills: Listen +8*, Spot +7*, and Swim +8
Advancement: 3-4 HD (Medium-size); 5-6 HD (Large)
Climate/Terrain: Temperate aquatic
Organization: Solitary or school (3-20)
Treasure/Possessions: none

Source: Monster Manual

Blindsight (Ex): Porpoises can "see" by emitting high-frequency sounds, inaudible to most other creatures, that allow them to locate objects and creatures within 120 feet. A silence spell negates this and forces the porpoise to rely on its vision, which is approximately as good as a human's.

Skills: Porpoises gain a +4 racial bonus to Spot and Listen checks. These bonuses are lost if blindsight is negated.