A horse, a horse, my kingdom for a horse

Origin
From Shakespeare's Richard III.
CATESBY: Rescue, my Lord of Norfolk, rescue, rescue!
The king enacts more wonders than a man,
Daring an opposite to every danger:
His horse is slain, and all on foot he fights,
Seeking for Richmond in the throat of death.
Rescue, fair lord, or else the day is lost!
KING RICHARD III: A horse! a horse! my kingdom for a horse!
CATESBY: Withdraw, my lord; I'll help you to a horse.