Fate's name, sprite and presence in the Locked Chest pool are an implication to Isaac's ultimate fate: to lock himself in the chest and become ???. This is likely why Fate was left in the Locked Chest pool even after head-themed items were removed from the pool.
Isaac's winged form when he picks up Fate is similar in many aspects to the winged forms of both his boss incarnation and that of ???.
In Wrath of the Lamb, this item was unlocked by default.
Eternal Heart.
Keeper will grant Blue Flies instead of the Eternal Heart.
Locked Chest pool are an implication to Isaac's ultimate fate: to lock himself in the chest and become 
???. This is likely why Fate was left in the Locked Chest pool even after head-themed items were removed from the pool.
??? (Character) Unlocks 
??? (Boss) Unlocks 
The Binding of Isaac: Rebirth
