Nyxborn Unicorn

If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.

On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.

Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its {T} abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.

If the creature with mentor leaves the battlefield with the mentor ability on the stack, use its power as that creature last existed on the battlefield to determine whether the target creature has lesser power.

Mentor compares the power of the creature with mentor with that of the target creature at two different times: once as the triggered ability is put onto the stack, and once as the triggered ability resolves. If you wish to raise a creature's power so its mentor ability can target a bigger creature, the last chance you have to do so is during the beginning of combat step.

Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.