Mogis's Warhound — ruling 1

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

Mogis's Warhound — ruling 2

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.

Mogis's Warhound — ruling 3

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.

Mogis's Warhound — ruling 4

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.

Mogis's Warhound — ruling 5

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.

Mogis's Warhound — ruling 6

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.

Mogis's Warhound — ruling 7

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.

Mogis's Warhound — ruling 8

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

Mogis's Warhound : règles, fonctionnement et interactions (MTG)

Mogis's Warhound is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Bestow (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.) This creature attacks each combat if able. Enchanted creature gets +2/+2 and attacks each combat if able.

How it works : Mogis's Warhound

Original English text — Wizards of the Coast / Scryfall

#1April 26, 2014· Wizards of the Coast
The controller of Mogis's Warhound or the creature it's enchanting chooses which player or planeswalker to attack.
No other card is named in this ruling.
#2April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#4April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#5April 26, 2014· Wizards of the Coast
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 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.
No other card is named in this ruling.
#6April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#7April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#8April 26, 2014· Wizards of the Coast
You still control the Aura, even if it's enchanting a creature controlled by another player.
No other card is named in this ruling.
#9April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#10April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.