Blue Jellyfish

They need water deeper than a single tile sitting on a solid block, and after that the biome picks which one you meet. Blue and green share the flooded caves anywhere below the surface layer, and they will generate down in the Underworld if somebody flooded it. Pink ones want sand inside the Ocean. Blood Jellies only require you to be standing in the Crimson, not the water to be sitting on Crimson blocks, so any ordinary flooded pocket inside the biome produces them. Fungo Fish need mushroom grass under the water. Once Hardmode starts the green variant takes over most of those spawns and the blue kind becomes noticeably scarce, so a wired Jellyfish Statue is the dependable way to get Blue Jellyfish on demand.

There is no ranged attack and no cleverness here. Underwater they shove themselves toward you in short bursts, coast, then shove again, which reads as harmless right up until three of them converge while you are also watching your breath meter. Everything they do is contact damage. Out of water they are inert. They will hop clear of the surface chasing you and land on dry stone, and there they sit, unable to move until water reaches them again, so knocking one onto land is a legitimate way to win. Most of them glow, which at least means an unlit flooded cave announces its jellyfish before you swim into one.

In Expert and above, every variant except the Fungo Fish periodically charges itself up. While charged it cannot be damaged at all, and it holds its speed in a straight line until it strikes a block or reaches the surface. Attacking it in that state punishes you instead: melee swings and a range of projectiles reflect damage back at you, and the hit lands harder than simply bumping into the thing would. Banners do not soften that particular damage, which catches people out in a farm they assumed was safe. The answer is to read the animation and stop attacking, or to knock the jellyfish out of the water first, because a stranded one cannot charge up at all.