Grants a 1/3 chance of replacing Room Clear Awards with a Chest or a Locked Chest, but also a 2/9 chance to replace it with nothing at all.
This item belongs to the Guppy set. Collecting three items from this set will transform Isaac into Guppy.
Notes
The drawback to this item is that Keys become much more scarce, while increasing the amount of Locked Chests. This makes taking Guppy's Tail a potential liability early on.
Due to any found Chests containing items while in Chest or Dark Room, Guppy's Tail can be a good way to quickly stack up more upgrades before facing the final boss.
The effect does not apply to Pennies dropped in Greed Mode, meaning that this item has no effect in Greed Mode other than counting towards the Guppy Transformation.
Synergies
Deep Pockets: If Guppy's Tail removes the room clear drop, it is turned into coins instead.
Guppy's Eye: Makes Key management easier by revealing which Locked Chests are worth opening and which are not.
The Left Hand: Turns all chests into Red Chests, allowing Isaac to get Red Chest items much more quickly. Useful when pursuing the Guppy transformation.
Mom's Key: Increases the number of drops from Chests and counteracts the increased chance to run into Locked Chests.
Options?: Chests may be accompanied by normal room drops or other chests of a type that's easier to open, mitigating the downsides.
Paper Clip: The Key scarcity problem is completely prevented, since Paper Clip causes Locked Chests to be openable without Keys, making Guppy's Tail's only downside the chance to replace room clears with nothing at all.
If Cain starts with the Trinket, entering Curse Rooms and opening Red Chests in hopes of finding Guppy's Tail early is a very effective strategy.
Chest or a 
Locked Chest, but also a 2/9 chance to replace it with nothing at all.
Guppy set. Collecting three items from this set will transform Isaac into Guppy.
Chest or 
Dark Room, Guppy's Tail can be a good way to quickly stack up more upgrades before facing the final boss.
The effect does not apply to 
Pennies dropped in Greed Mode, meaning that this item has no effect in Greed Mode other than counting towards the Guppy Transformation.
Deep Pockets: If Guppy's Tail removes the room clear drop, it is turned into coins instead.
Guppy's Eye: Makes 
Key management easier by revealing which Locked Chests are worth opening and which are not.
The Left Hand: Turns all chests into 
Red Chests, allowing Isaac to get Red Chest items much more quickly. Useful when pursuing the 
Mom's Key: Increases the number of drops from Chests and counteracts the increased chance to run into Locked Chests.
Options?: Chests may be accompanied by normal room drops or other chests of a type that's easier to open, mitigating the downsides.
Paper Clip: The Key scarcity problem is completely prevented, since Paper Clip causes Locked Chests to be openable without Keys, making Guppy's Tail's only downside the chance to replace room clears with nothing at all.
Cain starts with the Trinket, entering 
Curse Rooms and opening Red Chests in hopes of finding Guppy's Tail early is a very effective strategy.
Dead Cat has Guppy's tail be completely black while Guppy's Tail has a white fur on the tip of the tail.
Pet Items 
The Binding of Isaac: Rebirth
