Zombie Merman

This one has no spawn of its own. It exists purely as a fishing result during a Blood Moon: the bobber dips, you reel, and instead of a fish something climbs out of the water a few tiles away. The Wandering Eye Fish uses the same slot, so any bite on a Blood Moon night is a coin flip between loot and a fight. Nothing else about the world matters. You can meet one in the first pond you dug, before Hardmode, on a fresh character's first Blood Moon.

A Sonar Potion is close to mandatory. It names what is on the hook before you commit, so you can put the rod away and pick up a weapon, or simply leave the line alone if you would rather keep fishing. Going the other way, a Chum Caster raises how often blood moon enemies bite, which is what you want once you are hunting on purpose instead of being ambushed while chasing crates. That rod is one of its own drops, so the first kill tends to make the next ten easier.

On dry land it is clumsy and slow enough to walk away from. In the water it is a completely different animal, turning on the spot, closing instantly, and launching itself clean out of the surface at you. It also speeds up as its health falls, so the last quarter of the fight is the dangerous part, and it takes no knockback at all, which means there is no shoving it back while you reposition. For an underequipped character it hits hard and refuses to die.

The clean answer is to floor over your fishing hole with platforms. You can still cast through them and it cannot use the water, which strips away everything that makes it difficult. Note that it breaks down doors the way other Blood Moon walkers do, so a fishing shack is not a retreat.

For an early character this is one of the biggest power spikes a Blood Moon can hand out. Its drop table sits well ahead of the curve for pre-Hardmode, and the Bloody Tear is the item that compounds, since it lets you start the next Blood Moon on your own schedule instead of waiting for the game to offer one. Its Bestiary entry and banner also complete in half the usual number of kills, so a single night of fishing can finish it outright.

It looks like a drowned take on the merfolk shape the player takes with Neptune's Shell equipped. It is also one of the few things a Lifeform Analyzer bothers to announce, and its banner is the very last item in the game alphabetically.