Skum (CR 2)

Medium Aberration (Aquatic)
Alignment: Usually lawful evil
Initiative: +1 (Dex); Senses: darkvision 60 ft., Listen +7*, and Spot +7*
Languages: Aquan

AC: 13 (+1 Dex, +2 natural), touch 11, flat-footed 12
Hit Dice: 2d8+2 (11 hp)
Fort +1, Ref +1, Will +3
Speed: 20 ft., swim 40 ft.
Space: 5 ft./5 ft.
Base Attack +1; Grapple +5
Attack: Bite +5 melee
Full Attack: Bite +5 melee and 2 claws +0 melee
Damage: Bite 2d6+4, claw 1d4+2
Special Attacks/Actions: Rake 1d6+2
Abilities: Str 19, Dex 13, Con 13, Int 10, Wis 10, Cha 6
Special Qualities: amphibious
Feats: Alertness
Skills: Hide +6+, Listen +7*, Move Silently +3, Spot +7*, and Swim +12
Advancement: 3-4 HD (Medium-size); 5-6 HD (Large)
Climate/Terrain: Any underground
Organization: Brood (2-5) or pack (6-15)
Treasure/Possessions: None

Source: Monster Manual

In the water, skum are dangerous enemies who attack by biting, clawing, and raking with their rear legs. On land they are less dangerous, for they cannot rake and suffer a -2 circumstance penalty, to all attack rolls. Skum serving an aboleth are sometimes trained to fight with weapons, usually two-handed melee weapons with reach (such as long spears) and simple ranged weapons such as javelins, tridents, or slings.

Skills: Skum receive a +4 racial bonus to Hide, Listen, and Spot checks underwater.

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.