Hemogoblin Shark

Nothing summons it directly. It is a fishing hazard: during a Blood Moon in Hardmode, a bite on your line has a chance to be this instead of a fish, and it materializes a few tiles from your bobber the moment you reel in. The trigger is entirely under your control, which cuts both ways. Do not fish during a Blood Moon and you will never meet one. It is also one of the handful of enemies a Lifeform Analyzer will call out.

It is faster on the ground than you are. Terraspark Boots are not enough to walk away from it, which removes the default plan for every other awkward Blood Moon spawn. It swims as well as it runs, jumps high enough to reach most improvised perches, and fires volleys of blood shots at range. Lava does not slow it and neither does water. Burning it still works, as long as the fire comes from a weapon rather than from lava.

• It cannot fire while submerged. Keep it in the water and it is reduced to a contact damage enemy that has to reach you.
• It is four tiles tall, so a doorway stops it. It keeps shoving at the frame instead of routing around, which is a free kill if your weapon reaches through the gap.
• A platform well above your fishing spot takes the lunges away, though the ranged attack still climbs to you.
• Seal yourself in and use something that hits through blocks, like the Crystal Vile Shard, and it has no answer at all. Slow, but it never touches you.
• If it wedges itself in a crevice, the Flying Knife finishes it without you having to approach.

A Sonar Potion names what is on your line before you commit, and Blood Moon spawns are listed in red, so you get a moment to reposition or to simply not reel in. Going the other way, a Chum Caster makes these hookings much more common. The Bloody Tear is the drop worth the trouble, since it lets you start a Blood Moon on demand instead of waiting for one. Its banner also unlocks at half the usual kill count.

The name folds hemoglobin, the protein that colors blood, into goblin shark, a deep water species it clearly borrowed its face from. Hemogoblin is also an exact anagram of hemoglobin. Its bestiary line is built off the Minishark tooltip with the gun half swapped for a goblin, and it reuses every sound the Drakin makes, which is a decent hint about where it came from.