The king who cries “A horse! A horse! My kingdom for a horse!” is Richard III.
The line occurs in Act 5, Scene 4 of Shakespeare’s history play Richard III, during the Battle of Bosworth Field in 1485. Richard has been thrown from his horse while searching for Richmond, the future Henry VII. Desperate to continue fighting, he offers his kingdom in exchange for another mount.
The speech dramatizes Richard’s rapidly collapsing power. Shakespeare presents him as a ruthless ruler whose ambition has brought him to the battlefield, alone and increasingly desperate. Richmond defeats Richard soon afterward and takes the throne, beginning the Tudor dynasty.
A common mix-up is to attribute the line to King Lear because it sounds like a tragic royal lament. Lear does not say it. Macbeth and Henry V are also Shakespearean kings, but the quotation belongs specifically to Richard in Richard III. The wording is often shortened in modern usage to “My kingdom for a horse.”