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.
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.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Valera Lutfullina
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
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).
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.
