A bezoar is a ball of hair or vegetable matter that usually forms in the stomachs of cud-chewing animals (cows, goats, etc.), although humans sometimes get them as well. In Elizabethan and Colonial times, bits of bezoar were reputed to help cure poison.
The word derived from the Persian "pad-zahr," meaning counter-poison.
The bezoar really did have the ability to cure poison, and specifically came from goats' bellies. It was discussed in Harry Potter's first year in Professor Snape's Potion Class at Hogwarts School of Witchcraft and Wizardry.