Zombie Merman

The Zombie Merman is a pre-Hardmode enemy which can be spawned by fishing in a Blood Moon. When the bobber dips and a player clicks to reel in a fish, a Zombie Merman or a Wandering Eye Fish has a chance to spawn in place of a fish, within a few blocks to either side of the bobber. These two enemies drop various useful items, including the Vampire Frog Staff and the Chum Caster.

Tips
The Zombie Merman has high health and defense, and it also doesn't take knockback, so underpowered characters should beware.