Mogis's Warhound

Bestow {2}{R} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Mogis's Warhound attacks each combat if able.
Enchanted creature gets +2/+2 and attacks each combat if able.

Pioneer Masters

Journey into Nyx

Bestow {2}{R} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Mogis's Warhound attacks each turn if able.
Enchanted creature gets +2/+2 and attacks each turn if able.

An Aura that becomes a creature is no longer put into its owner's graveyard as a state-based action. Rather, it becomes unattached and remains on the battlefield as long as it's a creature. While it's a creature, it can't be attached to another permanent or player. An Aura that's not attached to a legal permanent or player as defined by its enchant ability and also isn't a creature will be put into its owner's graveyard as a state-based action.

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.

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.

If the enchanted creature leaves the battlefield, the Aura stops being an Aura and remains on the battlefield. Control of that permanent doesn't change; you'll control the resulting enchantment creature.

If, during your declare attackers step, Mogis's Warhound (or the creature it enchants) is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under its controller's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.

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.

Similarly, if you cast an Aura spell with bestow targeting a creature controlled by another player, and that creature is an illegal target when the spell tries to resolve, it will finish resolving as an enchantment creature spell. It will enter the battlefield under your control.

The controller of Mogis's Warhound or the creature it's enchanting chooses which player or planeswalker to attack.

Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.

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.

You still control the Aura, even if it's enchanting a creature controlled by another player.