Fire Koi

Absurd and impossible, the fire koi hunts on land, leaping through the deep mountain snow. Part koi carp and part lungfish, it has thick yellow-red scales, permanently erupting in bursts of flame, usually surrounded by a lake of slush and a cloud of scalding steam. It kills by crashing into its prey, latching on with its jaws and thrashing atop them with its searing scales. In summer months, when they sometimes emerge in snowless lowlands, they dry up springs, stampede yak herds and conflagrate villages. Know them by the scent of saltpetre and ashes on the wind.

Mandarins and other potentates prize the fire koi as the ultimate status symbol among fighting fish - massive and dangerous and difficult to keep, captured by desperate adventurers cajoled into service as hunters using nets of woven steel. 

The body has many uses for the brave and fire-retardant alchemist: Its meat remains warm for days after its butchered. Its ground scales make serviceable gunpowder. Its blood is prized as a potent unguent to heal frostbite and ward off biting cold. 

Black Hack /OSR 

HD 2, Armour chain (1), Move human, Attacks Bite D6, then Thrash 2d6, Morale 8. 

Into the Odd 

STR 14 DEX 12 WILL 10, HP 6, ARMOUR 1 

DRIVEN TO LEAP UPON PREY AND LOCK ITS JAWS ON THEM. Attacks with its latching bite, then thrashing and flames. Big enough to damage regiments.

D&D 5e

FIRE KOI

Large elemental, unaligned

AC 14 (natural armour) Hit Points 114 (12d10 + 36), Swim 40ft.

STR 17 (+3)   DEX 14 (+2)  CON 16 (+3)

INT 2 (- 4)     WIS 11 (+0)  CHA 5 (- 3)

Skills passive Perception 14

Challenge 5 (1, 800 XP)

Beacon: The fire koi’s flames shed bright light in a 30-foot radius

Steams in Snow: The Fire Koi’s flames turn snow and ice to mush and a cloud of blinding steam. Creatures within 10ft. of the fire koi.

Flameheart: The fire koi burns hotter when it’s exposed to water, eventually turning itself to ash rather than be doused. If submerged for a full round, it takes 1D10 damage.  

ACTIONS

Bite: Melee Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) fire damage

Thrash: The fire koi jumps up to 40 feet as its movement, landing on one or more other creatures. Each creature must succeed on a DC 16 Strength or Dexterity saving throw (target's choice) or be knocked prone and take 14 (2d6 +3) bludgeoning damage plus 10 (2d6 + 3) fire damage. On a successful save, the creature takes only half the damage, isn't knocked prone, and is pushed 5 feet out of the koi’s space into an unoccupied space of the creature's choice. If no unoccupied space is within range, the creature instead falls prone instead.