Mangon of Moraine
In Robert Biket’s Lai du Cor, the King of Moraine, he sent a horn to Arthur’s court which would expose any infedility on the part of his wife.
The horn spilled its contents on men with “unchaste” wives, which turned out to include every man at Arthur’s court (including Arthur) except Sir Caradoc.
See also
Chastity Test | The Legend of King Arthur
Source
Lai du Cor | Robert Biket, mid to late 12th century