Axolotl

Axolotl is the lush-cave amphibian that fights on your side underwater. Scoop one into a water bucket and it travels as an item; tip it out next to a guardian or a drowned and it goes to work on its own. There is no taming step to learn — an Axolotl cannot be tamed the way a wolf or a cat can, and the closest thing to it is a lead and the fact that one will follow anybody holding a bucket of tropical fish.

Those buckets are also the only breeding food; the loose fish item does nothing. A baby takes its color from one of its parents, with one exception: roughly one baby in 1200 comes out blue no matter what the parents look like. That makes the blue axolotl the rarest of the five colors and something you breed toward rather than something you find, because only the other four — pink, brown, gold, and cyan — ever appear in the world on their own.

Its real combat value is not the damage it deals. When a player finishes off a mob an Axolotl was attacking, the player gains Regeneration and has any Mining Fatigue stripped away, which makes a bucket of them the practical answer to an ocean monument. They attack most water mobs but leave turtles, dolphins, frogs, and each other alone, so a tank of axolotls is stable while the tropical fish next door are not.

They are also hard to kill. An injured Axolotl plays dead, healing while hostile mobs lose interest in it entirely. Out of water they run on a timer and start taking damage when it expires unless rain or a splash of water resets it, so moving one overland means carrying the bucket rather than walking it on a lead.