Flickerform and Bestow
When Theros was released, this rule was added to the Comprehensive Rules:
303.4h If an effect attempts to put a permanent that isn't an Aura, Equipment, or Fortification onto the battlefield attached to an object or player, it enters the battlefield unattached.
In this case, when the Eidolon returns to the battlefield, it will return as an enchantment creature, not an aura, and it will return unattached (it will not attempt to attach to the creature).
Nathan Long
DCI Level 2 Judge
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