Wandering Eye Fish

The Wandering Eye Fish is a pre-Hardmode enemy that can be spawned by fishing in a Blood Moon. When the bobber dips and the player attempts to reel in a fish, a Wandering Eye Fish or a Zombie Merman has a chance to spawn within a few blocks of the bobber in place of a fish.

Tips

 * The Wandering Eye Fish has high health and defense, moves very quickly, and doesn't take knockback, making it a challenging enemy for underpowered characters.
 * It is recommended to avoid fishing in a Blood Moon while inside an enclosed space (like a building), as this may make escape difficult if the player is caught off guard.
 * One way to effectively fight the Wandering Eye Fish is to obtain a movement accessory such as a Cloud in a Bottle or Rocket Boots and make tight circles around the eye. Its top speed can easily outrun early-game players, but its turning radius is limited.
 * Unlike most flying enemies, the Wandering Eye Fish can enter water, though it will still be slowed down when submerged.
 * If the Wandering Eye Fish is afflicted with the Confused debuff, it may fly away far enough to despawn unless the player chases it.
 * A way to easily beat the Wandering Eye Fish relatively early in the game is to stomp it with a Slimy Saddle. Jumping on the Wandering Eye Fish with the saddle on will make it vertically lock on to the player, and it will quickly melt its own health bar by ramming itself into the mount. The player should take care to avoid falling to their death if they miss a stomp.
 * Another way to easily beat the Wandering Eye Fish is to make a fishing pool inside a cage made of solid blocks of approx. 12*12 tiles, using an Bottomless water bucket and leaving a small space of air. With a pickaxe, extract 1 block from the ceiling and fish from it. Using a summon Weapon such as Abigail's flower or a slime staff can easily deal with the Wandering Eye Fish. You could also use a rapid-fire ranged weapon such as a Minishark to shoot bullets through the gap and stun-lock it, or a projectile-firing melee weapon such as the Ice Blade or Enchanted Sword, or Gem staves to deal with it.

Trivia

 * It has a similar appearance to Demon Eyes and Wandering Eyes, being named after the latter. Rather than severed nerves and blood vessels trailing from its rear, it sports red fins. It closely resembles The Fish of Cthulhu.