Ahuizotl

Imagine an alligator with long, feathery blue-green scales, its body held high on long legs rather than slung low. It has sharp teeth and long claws and breathes both air and water.

It rushes in with claws and teeth, and its tail slaps for 2d10. Underwater, landing both front claws lets it follow with two hind rakes of 1d4+1.

Three times daily, in place of other attacks, it spits a jet of water 5 feet wide and 15 feet long for 1d6 to anyone hit. The jet then comes alive as something close to a water weird, with AC 4, 3+3 HD, a move of 12, and hit points equal to a third of the ahuizotl's, rounded up. It strikes once a round with THAC0 15, as a monster of 6 HD, can hold only a single target, and pulls that victim under; each round the victim must save vs. paralyzation or drown. The ahuizotl directs it by thought.

Purify water kills the weird. Cold slows it, and fire deals either half damage or none, based on a save made at the ahuizotl's level. Sharp weapons inflict just 1 point, but at 0 hit points it splashes into plain water for good. It cannot take over water elementals, lasts 5d4 rounds even if its creator dies, and is worth 420 XP. A losing ahuizotl often flees behind its weirds.

It prefers fresh water, ideally a flooded sinkhole, and torments nearby villagers until they toss in food, art and treasure, live animals most of all. Raid them too often and it may start demanding people. Fortunately the species is dying out.

Pairs meet only to breed. Some 2 weeks later the female lays 1d4 eggs that hatch in about 12 weeks, and both parents guard them and hoard for them, yet predators and irritated parents leave only one or two to mature. Hatchlings have 5 HD and attacks of 1d3/1d3/3d3/1d10, and grow up in a year. Mates usually part after that year, often violently.

Few water predators are as feared; crocodiles and alligators rarely beat its strength and wits. Treasure should use local values, with coral buds instead of coins. Its scales make decorative armor equal to scale mail.