Breath of Fury attached to any other Horse you control with lesser base power than Shadowfax's power.
You have a Horse creature card in hand with lesser base power than Shadowfax's power.
An opponent cannot block creatures you control.
Steps
Declare Shadowfax and the Horse with Breath of Fury attached as attackers.
Shadowfax triggers, putting a Horse creature card from your hand onto the battlefield tapped and attacking.
Deal combat damage using Shadowfax and the two other Horses.
Breath of Fury triggers, causing you to sacrifice the Horse it is attached to, then attach Breath of Fury to another Horse except Shadowfax, then causing you to untap all creatures you control and to get an additional combat phase after this one.
When the Horse that had Breath of Fury attached dies, Enduring Renewal triggers, returning the Horse to your hand from your graveyard.