Cryohydra, Eleven-Headed (CR 12)

Huge Magical Beast (Cold)
Alignment: Usually neutral
Initiative: +1 (Dex); Senses: darkvision 60 ft., low-light vision, scent, Listen +9, and Spot +9

AC: 15 (-2 size, +1 Dex, +6 natural), touch 9, flat-footed 14
Hit Dice: 11d10+58 (118 hp)
Fort +12, Ref +8, Will +5
Speed: 20 ft., swim 20 ft.
Space: 15 ft./10 ft.
Base Attack +11; Grapple +25
Attack: 11 bites +16 melee
Full Attack: 11 bites +16 melee
Damage: Bite1d10+6
Special Attacks/Actions: Breath weapon
Abilities: Str 23, Dex 12, Con 20, Int 2, Wis 10, Cha 9
Special Qualities: fast healing 21, cold immunity, vulnerable to fire
Feats: Blind-fight; Combat Reflexes; Iron Will; Toughness; Weapon Focus (bite)
Skills: Listen +9, Spot +9, and Swim +14
Advancement: -
Climate/Terrain: Cold marsh and underground
Organization: Solitary
Treasure/Possessions: 1/10 coins, 50% goods, 50% items

Source: Monster Manual

Hydras can attack with all their heads at no penalty, even if they move or charge during the round.

These hydras can breathe jets of frost 10 feet high, 10 feet wide, and 20 feet long. All heads breathe once every 1d4 rounds. Each jet deals 3d6 damage per head. A successful Reflex save halves the damage. The save DC is 10 + 1/2 hydra's original number of heads + the hydra's Constitution modifier.

Cold Subtype (Ex): Cold immunity; double damage from fire except on a successful save.

Skills: Hydras receive a +2 racial bonus to Listen and Spot checks, thanks to their multiple heads.

Feats: A hydra's Combat Reflexes feat allows it to use all its heads for attacks of opportunity each round.

Cold Subtype

A creature with the cold subtype has immunity to cold. It has vulnerability to fire, which means it takes half again as much (+50%) damage as normal from fire, regardless of whether a saving throw is allowed, or if the save is a success or failure.