Guppy's Hairball

Guppy's Hairball is an unlockable passive item.

Effects

Spawns a hairball familiar that loosely follows behind Isaac, blocks shots, and can be swung like a flail to damage enemies.
- The ball deals 5 damage per tick, but grows, increasing damage by 2 each time it kills an enemy — up to 5 times for a maximum of 13 damage. The damage it deals per tick (each half-second) is (level × 2) + 3.
- The hairball level decreases upon entering a new floor: if level 2, it reverts to level 1. If level 3 or 4, it reverts to level 2. If level 5, it reverts to level 3.
This item belongs to the Guppy set. Collecting three items from this set will transform Isaac into Guppy.

The hairball's hitbox is the same size regardless of level.
If Isaac has transformed into Guppy, Guppy's Hairball spawns Blue Flies when it deals damage.
If Blindfolded, the ball occasionally spawns a Blue Fly while it's damaging an enemy.
The hairball will be reset to level 1 after entering and leaving the Mother's Shadow escape sequence.

This item is used in Cat Got Your Tongue (challenge #8).
The function and flavor text of this item might reference the idioms "no room to swing a cat" and "can't swing a dead cat without hitting...".
In the original The Binding of Isaac, Guppy's Hairball did not count towards the Guppy transformation.