Missingno

Missingno is a pre-hardmode boss that looks like some kind of error. It can be summoned using the Mysterious Matrix during day or night in the underworld.

Behavior
Missingno just chases after the character at high speeds, leaving a trail of a deadly dust that does around 75 damage.

Known Bugs
It is the bug (In all seriousness, no known bugs)

Trivia
[https://en.wikipedia.org/wiki/MissingNo. MissingNo, also known as MISSINGNO. or MissingNO, short for Missing Number, is an unofficial Pokémon species found in the video games Pokémon Red and Blue. Due to the programming of certain in-game events, players can encounter MissingNo. via a glitch.]