Leech

The Leech is a wormlike enemy summoned by the Wall of Flesh.

It is released from the middle segment of the Wall of Flesh, the mouth part. It is at first spewed out in groups of 3-4, but the groups become larger, and increasingly faster as the boss takes damage - it reaches a point where the Wall of Flesh is almost constantly vomiting up leeches.

Leeches are a major part of the Wall of Flesh's damage per second, and should be dealt with as soon as possible. They will always drop a heart when they die, and are fairly easy to kill, so they can actually be useful by helping you regain your health during the boss fight.

Leeches don't despawn once the Wall of Flesh has been defeated.