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 there is a point where the Wall of Flesh is almost constantly vomiting up leeches.

They are considered 'worm-like' not because of any specific attribute, but because they're aggravatingly fast and extraordinarily hard to hit.

Leeches are major damage dealers of the Wall of Flesh, and should be dealt with as soon as physically possible.