Angler Fish

No summon and no event. It spawns on its own in Hardmode, in the Jungle and down at Cavern depth, which in practice means you meet it in a flooded tunnel with nowhere to back up to. The glowing bulb on its head throws off a little light, so a faint glow in dark water is usually the only warning you get.

The bite hurts less than what it leaves on you. Bleeding shuts off health regeneration, and it lasts noticeably longer in Expert and longer again in Master, so you surface with a bar that will not refill until it expires or you burn an Adhesive Bandage. Losing regen while you are also on a breath timer is what actually kills people down there.

It swims faster than the Piranhas you have been ignoring, so retreating through the water is not a plan. Pull it somewhere you can stand, drain the pool from underneath, or carry a weapon that still works submerged. Any breathing accessory makes the encounter trivial, because without the drowning clock you can simply turn around and deal with it.