Nyxborn Hydra

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  tap  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 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.