The bezoar is a hard ball of hair or vegetable fibre that occurs in the stomachs of cud-chewing animals such as goats (though humans get them, too). If you feel like categorising them, a trichobezoar is a hairball, while a phytobezoar is one that contains mostly vegetable fibres.
The word is Persian (pad-zahr, counter-poison or antidote) and the bezoar’s fame as a cure for poison spread westwards from there in medieval times. You swallowed it, or occasionally rubbed it on the infected part. In A Voyage to Abyssinia, written by Father Lobo in the eighteenth century, he says: “I had recourse to bezoar, a sovereign remedy against these poisons, which I always carried about me”. Belief in its near-magical properties was then common.
Old herbals are full of recipes using it, such as this one from Nicholas Culpeper’s Complete Herbal of 1653: “Take of Pearls prepared, Crab’s eyes, red Coral, white Amber Hart’s-horn, oriental Bezoar, of each half an ounce, powder of the black tops of Crab’s claws, the weight of them all, beat them into powder, which may be made into balls with jelly, and the skins which our vipers have cast off, warily dried and kept for use”. Culpeper remarks that “four, or five, or six grains is excellently good in a fever to be taken in any cordial, for it cheers the heart and vital spirits exceedingly, and makes them impregnable”. Don’t try this at home!
(It’s not quite as bad as it sounds; scrapings of hart’s horn were frequently used as a thickener for jellies, and crab’s claws was a common British water plant.)