Sand Shark

These only appear in Hardmode, only in a Desert, and only while a Sandstorm is blowing. The desert also has to have real depth to it. A thin skin of sand over stone produces nothing, though the depth requirement is relaxed in Remix worlds. That combination means you cannot simply walk into a desert and expect one. Either wait for the weather to turn or start your own with a Sandstorm in a Bottle and spend it somewhere sandy.

They travel inside the ground rather than on top of it. Sand, Hardened Sand and Sandstone are all open water to them, and every other block stops them cold. While buried they give almost no warning, and the attack is a lunge that bursts out from under you once you get close. Standing on open sand is the worst place to be. Fight from a platform or a stone floor and the ambush disappears entirely, because they cannot reach you through material they do not swim in. One that ends up in actual water drops the burrowing behavior and comes at you like an ordinary Shark.

There is a strict checklist attached to them, and most failed farms fail on it.

• Any town NPC nearby shuts spawning down, so a setup built beside your base produces nothing.
• A safe background wall behind your character does the same.
• The game also wants you to have done something recently: moved, jumped, pressed a direction, or used an item that is not a kite. Standing perfectly still stops the spawns, and riding a conveyor belt does not count as moving.
• Holding a Shadow Candle, or standing near a placed one, lifts all of it at once. That is the fix for an AFK setup.

Which version you get depends on the sand block it came out of, not on the biome around it. Ebonsand gives you a Bone Biter, Crimsand a Flesh Reaver, and Pearlsand a Crystal Thresher, even if the surrounding desert is plain. That makes hand-built spawn boxes easy: lay whichever sand you want and wall the box in with solid blocks that are not sand, so nothing can burrow out through the floor. They drop Shark Fin that looks different from the ocean version but stacks with it, plus Nachos, and on Windy Days the Sand Shark Kite.