Mantle of the Ancients

Enchant creature you control
When Mantle of the Ancients enters, return any number of target Aura and/or Equipment cards from your graveyard to the battlefield attached to enchanted creature.
Enchanted creature gets +1/+1 for each Aura and Equipment attached to it.

Fallout

Enchant creature you control
When Mantle of the Ancients enters the battlefield, return any number of target Aura and/or Equipment cards that could be attached to enchanted creature from your graveyard to the battlefield attached to enchanted creature.
Enchanted creature gets +1/+1 for each Aura and Equipment attached to it.

Wilds of Eldraine Commander

Enchant creature you control
When Mantle of the Ancients enters the battlefield, return any number of target Aura and/or Equipment cards from your graveyard to the battlefield attached to enchanted creature.
Enchanted creature gets +1/+1 for each Aura and Equipment attached to it.

As the enters-the-battlefield ability is resolving, any targets that couldn't be legally attached to the creature enchanted by Mantle of the Ancients stay in your graveyard.

If Mantle of the Ancients becomes attached to a different creature in response to its enters-the-battlefield ability, the creature it's attached to as that ability is resolving is the "enchanted creature," even though you used the original enchanted creature to choose targets. This means some of those targets may become illegal.

If Mantle of the Ancients leaves the battlefield before its enters-the-battlefield ability resolves, the last permanent it was attached to before leaving the battlefield is the "enchanted creature" (even if that permanent is no longer a creature).