Shakespeare’s Richard III was not the first Elizabethan play written about the latest Plantagenet king of England. An anonymous play, The True Tragedy of Richard III, was printed in 1594, though it’s thought to have been written and performed several years earlier. There was even a Latin play by Thomas Legge, Richardus Tertius, which was acted by the students of St. John’s College, Cambridge in 1579.
But it’s Shakespeare’s portrayal of the crookbacked schemer Richard III that has endured, with the play giving us such classic lines as ‘Now is the winter of our discontent’ and ‘A horse! A horse! My kingdom for a horse!’
This latter quotation is instantly recognisable to people who have never read or studied the play or even seen it performed. What most people aren’t aware of is that Richard actually utters the line twice. The battle is lost, and the king has lost his horse, as Catesby tells us:
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!
Alarums. Enter KING RICHARD III
KING RICHARD III:
A horse! a horse! my kingdom for a horse!
CATESBY:
Withdraw, my lord; I’ll help you to a horse.
KING RICHARD III:
Slave, I have set my life upon a cast,
And I will stand the hazard of the die:
I think there be six Richmonds in the field;
Five have I slain to-day instead of him.
A horse! a horse! my kingdom for a horse!
Exeunt
Although he has lost his horse, Richard fights on valiantly on foot, seeking out the Earl of Richmond, his foe: the man 
who will become King Henry VII when Richard is slain on the battlefield. In the next scene, Richmond will announce that Richard III has been slain and ‘the bloody dog is dead’. Victory is his, and the Tudor dynasty is born.
Where did Shakespeare get the horse from? Did he come up with it himself? No: as so often, he took from his source material, but in the taking he made it better. George Peele, in his play The Battle of Alcazar, has his Moorish character Muly Mahomet say:
A horse, a horse, villain a horse
That I may take the river straight and flie.
Peele (1556-96) is thought to have collaborated with Shakespeare on the bloody Roman tragedy Titus Andronicus (although some scholars, such as Jonathan Bate, have made the case for that play being solely Shakespeare’s work). At any rate, it’s highly probable that Shakespeare knew Peele’s work and may have been familiar with this line. It’s thought that The Battle of Alcazar, although only printed in 1594, was written perhaps five years earlier, possibly in the wake of the defeat of the Spanish Armada in 1588.
It is easy to see how Shakespeare latched onto the pleasing rhythm of Peele’s line, with its triple-repetition of ‘a horse’ (‘A horse, a horse … a horse’), and made it a comment on Richard’s desperation in the moment of battle. For he knows that he cannot win the battle on foot against cavalry fighters. His kingdom won’t be his kingdom for long unless he finds a horse and is able to ride to victory.
This is not the only interpretation. We often assume that Richard is offering to give away his kingdom to anyone who can furnish him with a horse; but we should remember that he is king, and we may interpret the line less as hyperbolic desperation than royal command: in other words, Richard is calling for one of his loyal servants or subjects to bring him a horse because, through doing so, they will help their king to save his kingdom.
But of course, one of the reasons the line ‘A horse! A horse! My kingdom for a horse’ has become so famous is Richard’s (assumed) hyperbole: the idea that he wants a horse so desperately that he would trade his whole kingdom for one is almost laughable.
Curiously, although we find a very close precursor to Shakespeare’s line in Peele’s play about a Spanish battle, Peele himself may have been inspired by a passage in Edward Hall’s chronicle, The Union of the Two Noble and Illustre Families of Lancastre and Yorke, published in 1548:
They began∣ne to suspect fraude and to smell treason, and not only exhorted but de∣terminatly aduysed hym to saue hym selfe by flyght: and when the losse of the battayle was imminent and apparante, they brought to hym a swyfte and a lyght horse to conuey hym awaie.
However, Hall goes on to note that Richard rejected this idea, since he was determined to ‘make an end of all battles or else there finish his life’. So in the original chronicle, the horse was brought to Richard in order to spirit him away from the battle to live another day; but in Shakespeare’s version, Richard calls for a horse, not so that he might make an escape but so that he might indeed, as in the chronicle, bring the war to an end of die in the attempt. (In the end, both happened – or almost. Contrary to popular belief, the Battle of Bosworth Field of 22 August 1485 was not the last battle in the Wars of the Roses: that was the Battle of Stoke Field in 1487, when the Yorkist sympathiser, the Earl of Lincoln, was killed, having failed to get Lambert Simnel, who was masquerading as one of the Princes in the Tower, crowned as king.)
Hall’s passage may in turn have given rise to Richard’s utterance in the anonymous Richard III play which predates Shakespeare’s, The True Tragedy of Richard III:
A horse, a horse, a fresh horse.
This preserves the pattern of Peele’s The Battle of Alcazar, that triple ‘a horse … a horse … a horse’, with the third iteration adding to the simple call for ‘a horse’. However, the text we have of this earlier Richard III may not be reliable, and it’s possible the version that was printed was a copy cobbled together from (faulty) memory, with Shakespeare’s (by then already famous) line incorporated into the text of the anonymous earlier play.
But it is only in Shakespeare that we find the whole ‘kingdom’ being offered for a horse. And it appears that the line quickly became ripe for parody and allusion: John Marston, in his 1598 book of satires The Scourge of Villanie, gives us ‘A man, a man, my kingdome for a man’. One can imagine it becoming something of a catchphrase in Elizabethan London, as people replaced ‘horse’ with whatever it is they happened to need at that particular moment.