Weaver
Weavers are worms that constantly weave in and out of the ground with large leaps as they chase Isaac. They will only deal contact damage while above the ground. They will burrow through pits and underneath obstacles, and while in pits or under obstacles, they cannot leap out of the ground until they reach an empty spot. They can also sometimes land on top of obstacles after leaping, in which case they will slide on top of them until they reach the end and immediately burrow back down.
Variants
Archer
Archers are Weavers which resemble 
Wormwood, and behave very differently from standard Weavers. An Archer will emerge halfway from a pit before firing a small cluster of projectiles towards Isaac. It will then jump towards Isaac, firing 4 projectiles in the cardinal directions on landing and getting stuck for a moment before eventually burrowing down into the ground and then appearing from another pit. If it lands in a pit when jumping, it will submerge again and will not fire any projectiles.
Weaver Sr.
Weavers Sr. behave identically to Weavers, but travel much faster.
Thread
Threads are Weavers that resemble 
Scolex and can only be damaged from their tail. After jumping, their lower half will get stuck in the ground while burrowing down and scatter 2 clusters of 3 short-ranged bloody projectiles to its left and right while also providing a moment of vulnerability. They will leap at half the rate of regular Weavers due to getting stuck every jump.
Diagetic
Diagetics are a rotten variant of Weavers that travel much faster and spend more time burrowing between each leap. Upon each leap, it will send out a line of 4 floating spectral projectiles that follow its movements for a few moments before it gets flung away in the direction the Diagetic was traveling. The line of projectiles will then propel itself towards Isaac's position, turning to loosely follow his movements, before eventually flying off-screen.
Dread Weaver
Dread Weavers burrow after Isaac extremely quickly and will make even larger leaps. While burrowing, they will create a damaging trail of rocks above them. This rock wave will destroy any obstacles they burrow under.
Upon death, they will shoot out a ring of 8 projectiles.
Trivia
- Weavers visually resemble
Needles from the official DLC, Repentance, as both enemies are based on the boss 
Pin.
- Needles were not an inspiration to Weavers, as Fiend Folio was released before Needles had been teased.
- Weavers, Weavers Sr., Diagetics, and Dread Weavers will occasionally have a different expression when burrowing out of the ground.
- An unused variant of a Weaver referred to as a "Strand" can be found in the game files. It heavily resembles
Slinger and even has a rare alternate sprite that uses Slinger's face. - Weavers and Weavers Sr. will each have unique appearances when encountered on the Glacier and Tomb floors respectively if the Revelations MOD is installed.
- Diagetic was added in The Spookaween Update.
- Archers have a unique appearance when encountered on the Boiler floor if the Fall from Grace MOD is installed.
Gallery
A Weaver with a different expression.
An Archer emerging from a pit.
An Archer stuck in the ground.
Archer's appearance in Dross.
A Weaver Sr. with a different expression.
A Thread stuck in the ground.
A Diagetic with a different expression.
A Dread Weaver with a different expression.
Weaver's appearance in Glacier.
Weaver Sr.'s appearance in Tomb.
Archer's appearance in Boiler.
