Nautilus

Nautilus is an ocean-dwelling shelled mob modeled on the real cephalopod, and it moves the way the animal does — mouth and tentacles trailing behind the direction it is swimming. It minds its own business until something provokes it: a struck adult hits back briefly before settling down again, and an untamed one will dash at any pufferfish it notices. Babies never turn on you, and neither do tamed adults, so a settled population is safe to build around.

Pufferfish are also how you tame a nautilus. Offer one the fish, or a bucket of it, and roughly every third attempt takes. From there a saddle turns it into an underwater mount you can ride, with a dash of its own and the Breath of the Nautilus effect, which stops the rider drowning while mounted. A tamed one will also wear nautilus armor, and any saddle or armor it is carrying drops if it dies.

Keeping them is straightforward: fish and fish buckets heal a tamed adult, and two adults at full health will breed. One detail matters for anyone living by the sea — a wild nautilus counts against the water-creature mob cap only until you interact with it, after which it becomes persistent and stops competing for spawn slots, so a handful of them will not choke the ocean around your base.

The drop worth hunting is the nautilus shell, which appears on only a small fraction of kills. Shells are the bulk ingredient for a conduit, so an underwater build needs a steady supply and the wild population is the way to get one. Nautiluses are immune to Poison, and in Java Edition the Impaling enchantment bites into them as it does any other aquatic mob.