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).