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Horses you control have haste. (They can attack and {T} as soon as they come under your control.)
Whenever Shadowfax, Lord of Horses attacks, you may put a creature card with lesser power from your hand onto the battlefield tapped and attacking.
Although the creature you put onto the battlefield is an attacking creature, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it enters the battlefield attacking.
Haste reminder text isn't normally included in most card sets these days, but we felt explaining what haste means was important for this card.
If Shadowfax, Lord of Horses leaves the battlefield before its triggered ability resolves, compare the power of the creature card in your hand with Shadowfax's power as it last existed on the battlefield.
The creature card you put onto the battlefield needs to have lesser power in your hand than the Shadowfax, Lord of Horses on the battlefield. The creature that enters the battlefield doesn't need to have lesser power than Shadowfax (this can happen if it enters with +1/+1 counters on it, for example).
You choose which player, planeswalker, or battle the creature you put onto the battlefield is attacking as it enters the battlefield. It doesn't have to be attacking the same player, planeswalker, or battle that Shadowfax, Lord of Horses is attacking.
