Can be picked up as The Soul half of The Forgotten to remove all but 1 Soul heart and keep all Bone Hearts.
Grants 9 extra lives.
Upon death, Isaac respawns in the previous room with 1 Red Heart container.
The head of Guppy begins following Isaac like a familiar upon pickup but doesn't do anything.
When Isaac has lost all of the lives provided by this item, the Guppy familiar will disappear.
This item belongs to the Guppy set. Collecting three items from this set will transform Isaac into Guppy.
Notes
This item can drop from killing Super Pride. Super Pride can drop this item even if Isaac has already encountered/possessed it.
Resurrection items activate in a set order. Dead Cat is 2nd / 4th in this order after Soul of Lazarus, 1up!, and Lazarus' inherent extra life.
Picking this item up as The Lost does not show the number of lives remaining, but still functions normally.
If picked up as The Forgotten, his Bone Hearts are reduced to 1 but The Soul's Soul Hearts remain unaffected; It is also true the other way.
The lives are added after the "picking up an item" animation has ended, so characters that die while the animation is playing will not be revived by this item.
The effects of the D4, D100, Missing No., or a 1- or 6-pip Dice Room will not remove the lives granted by this item.
Additional Dead Cats can be picked up during a run, increasing lives well past 9. Only the first dead cat counts toward becoming Guppy.
This item counts as 9 items, so if Isaac is trying for an early portal to The Void after beating Mom, this will significantly boost the progress closer to 20 items.
If Jacob and Esau take the item, only the brother who takes it will be set to one Red Heart Container on pickup. But if they die, both are resurrected with one Red Heart Container.
If both brothers take the item (via Diplopia or Birthright), resurrections will be subtracted from Jacob before Esau, regardless of which brother actually died.
Tainted Jacob's health will still be reduced to 1 Red Heart Container if he takes this item after being touched by Dark Esau or if he dies in his Lost form.
Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with Dead Cat having the lowest priority at 6th / 8th and thus being farther from Isaac than familiars of different priorities.
During the The Beast fight, Isaac is respawned on the spot upon death, as opposed to doing so in a previous room.
Despite the life counter being shown during the Mother's Shadow chase, the item is inactive like all other items and will not provide extra lives.
If all 9 lives are used, Dead Cat disappears from Isaac's inventory. If Isaac only has three Guppy items prior to losing Dead Cat, then its disappearance will cause the Guppy transformation to be lost.
Dead Cat's quality is treated as 2 for the purposes of Bag of Crafting.
Taking Dead Cat with 1 or 2 Heart Containers won't kill Isaac because it will set his number of Heart Containers to 1 immediately, saving him from the death that would have resulted from having no Heart Containers. Buying Dead Cat when at 1 Heart Container essentially obtains the item for free.
Other Devil Room items can be bought without the necessary number of Heart Containers at the cost of one of the many lives Dead Cat provides.
To take multiple Devil Deals, buy a Devil deal item then quickly leave the room before the "item get" animation finishes. This will result in Dead Cat reviving Isaac back inside the Devil Room.
Taking Dead Cat as Keeper or Tainted Keeper will reduce their total number of Coin Hearts to one, meaning they will die upon taking any damage at all. Unless either of them have immediate access to an HP Up item, such as from the Boss Item Drop, taking Dead Cat is usually detrimental.
Dead Cat no longer allows Keeper to spend his lives for Devil Deals like others characters due to his Devil Deal purchasing mechanic. As he no longer needs heart containers to purchase Devil Deals, it is more likely he will have extra heart containers around that can be used to mitigate Dead Cat's penalty.
Holding onto a XIX - The Sun? can be beneficial if at high HP. Using it prior to taking Dead Cat will convert all red heart containers to Bone Hearts for the floor. The bone hearts do not disappear when taking Dead Cat and will all revert back to normal red-heart containers on the next floor (and stay filled if the bone hearts were filled).
Synergies
Gold Hearts: Gives Isaac many coins when picked up, due to the amount of hearts lost.
Guppy's Paw: In addition to contributing to the transformation, converting the Red Heart Container obtained after revival into three Soul Hearts effectively triples Isaac's HP.
The Parasite - Pick up 2 dead-related items in a single run.
Gallery
The Dead Cat familiar in-game.
The Dead Cat familiar with Isaac.
Trivia
The 9 lives gained by the item is a reference to a myth that cats have 9 lives.
The item Guppy's Tail and the transformation Guppy implies that Guppy has a white fur on it's tip of the tail while the sprite for Dead Cat does not.
The Dead Cat shares the same tuxedo type bicolor fur with Guppy.
Despite not having Guppy in its name, this item counts for the Guppy transformation which may imply that Dead Cat is Guppy, or at least was what Guppy looked like at some point.
Bugs
Bug!
If picked up as ???, Black Judas, Tainted Judas, Tainted ???, Tainted Forgotten, or Tainted Bethany while in possession of Bone Hearts, the health bar may become glitched. The actual health amount is correctly set to 1 Soul Heart/Black Heart + all Bone Hearts, but some of the Bone Hearts are incorrectly displayed as half Soul Hearts depending on the situation before picking up Dead Cat. This can mess with the order of Soul Hearts and Bone Hearts after taking damage or gaining health. The glitch will resolve itself after taking damage, picking up health or touching an item.
Soul Hearts or 
Black Hearts unless picked up as 
???, 
The Soul, 
Black Judas, 
Tainted Judas, 
Tainted ???, 
Tainted Forgotten, or 
Tainted Bethany.
Bone Hearts unless picked up as 
The Forgotten.
Guppy set. Collecting three items from this set will transform Isaac into Guppy.
Super Pride. 
Super Pride can drop this item even if Isaac has already encountered/possessed it.
Soul of Lazarus, 
1up!, and 
Lazarus' inherent extra life.
Picking this item up as 
The Lost does not show the number of lives remaining, but still functions normally.
D4, 
D100, 
Missing No., or a 1- or 6-pip 
Dice Room will not remove the lives granted by this item.
This item counts as 9 items, so if Isaac is trying for an early portal to 
The Void after beating 
Mom, this will significantly boost the progress closer to 20 items.
Jacob and Esau take the item, only the brother who takes it will be set to one Red Heart Container on pickup. But if they die, both are resurrected with one Red Heart Container.
Diplopia or 
Birthright), resurrections will be subtracted from Jacob before Esau, regardless of which brother actually died.
Tainted Jacob's health will still be reduced to 1 Red Heart Container if he takes this item after being touched by 
Dark Esau or if he dies in his Lost form.
6th / 
8th and thus being farther from Isaac than familiars of different priorities.
The Beast fight, Isaac is respawned on the spot upon death, as opposed to doing so in a previous room.
Mother's Shadow chase, the item is inactive like all other items and will not provide extra lives.
Bag of Crafting.
Devil Room:
Keeper or 
Tainted Keeper will reduce their total number of Coin Hearts to one, meaning they will die upon taking any damage at all. Unless either of them have immediate access to an HP Up item, such as from the Boss Item Drop, taking Dead Cat is usually detrimental.
XIX - The Sun? can be beneficial if at high HP. Using it prior to taking Dead Cat will convert all red heart containers to 
Gold Hearts: Gives Isaac many coins when picked up, due to the amount of hearts lost.
Guppy's Paw: In addition to contributing to the transformation, converting the Red Heart Container obtained after revival into three Soul Hearts effectively triples Isaac's HP.
The Parasite - Pick up 2 dead-related items in a single run.
The Dead Cat familiar in-game.
The Dead Cat familiar with Isaac.
Guppy's Tail and the transformation 
Bug!
Black Judas, 
Tainted Judas, 
Tainted ???, 
Tainted Forgotten, or 
Tainted Bethany while in possession of 
Pet Items 
