Swamp Thing

Swamp Thing is a Hardmode enemy that spawns only during a Solar Eclipse, during which they are as common as a Zombie. They are immune to the Poisoned and Venom debuffs. It moves extremely fast, so it is recommended to use rapid-fire weapons or weapons with high knockback.

Trivia
"Swamp Thing" is a direct reference to the popular DC comic book-character. It even resembles him.