Crawdad

A Crawdad patrols the cavern floor and closes with short hops once you are inside its range. It moves quicker than the Crab it is built from, and unlike most ground enemies it keeps full speed through water, so flooding a corridor does nothing to slow one down. The part that actually kills you is the pincer swing it uses at point blank range, which hits for far more than a body bump.

That swing has conditions attached. The Crawdad has to be settled vertically, so neither rising nor falling, and it has to be underground. Above the surface layer it can only use the claw after dark. Anything that keeps it off the ground denies the attack outright and leaves it doing contact damage only, so heavy knockback or a fight over broken terrain takes most of the threat away.

Crawdads share a spawn pool with Giant Shellies and Salamanders. Every world picks two of the three at generation and shuts the third out permanently. If you have already run into both of the others down in your caves, no Crawdad will ever appear in that world, and nothing you do in game changes the assignment.

The game does account for this instead of punishing you for it. Once the two your world does have are recorded, the missing entry completes on its own in the bestiary, and any two banners from the trio can be combined into the banner of the third. A Crawdad free world can still finish the set.

There are two color variants and they behave identically, so there is nothing to read into which one you are looking at. The banner art and the bestiary portrait both show the red version regardless of which you killed.