A mounted knight in black armour created by the shore leave planet in 2267, which ran Dr McCoy through with a lance when he refused to believe it was real.
The Black Knight appears in the episode Shore Leave. After Yeoman Barrows finds a medieval princess's gown and puts it on, the thought of knights and maidens conjures a horseman in full black armour at the edge of the glade.
McCoy, convinced the visions are hallucinations that cannot hurt anyone, steps in front of Barrows and stands his ground as the rider lowers his lance and charges. The lance strikes him in the chest and he falls, apparently dead.
Kirk brings the knight down with the antique police revolver Sulu had found. Examining the body, Spock and Kirk find it is not human at all but a sort of mannequin made of the same cellular material as the planet's vegetation, proof that everything there is manufactured. McCoy is later restored unharmed.