Lucius Flavius Philostratus
Life of Apollonius of Tyana 3:2
English translation (F. C. Conybeare, 1912)
And they say that wild asses are also to be captured in these marshes, and these creatures have a horn upon the forehead, with which they butt like a bull and make a noble fight of it; the Indians make this horn into a cup, for they declare that no one can ever fall sick on the day on which he has drunk out of it, nor will any one who has done so be the worse for being wounded, and he will be able to pass through fire unscathed, and he is even immune from poisonous draughts which others would drink to their harm. Accordingly, this goblet is reserved for kings, and the king alone may indulge in the chase of this creature.
And Apollonius says that he saw this animal, and admired its natural features; but when Damis asked him if he believed the story about the goblet, he answered: "I will believe it, if I find the king of the Indians hereabout to be immortal; for surely a man who can offer me or anyone else a draught potent against disease and so wholesome, will he not be much more likely to imbibe it himself, and take a drink out of this horn every day even at the risk of intoxication? For no one, I conceive, would blame him for exceeding in such cups."