Muggles' Guide to Harry Potter/Magic/Bezoar

Overview
A bezoar is a stone (actually a ball of compacted hair) that is removed from the stomach of a goat, and the ingestion of such a stone is said to cure most poisons.

Extended Description
We are first introduced to this artifact when Professor Snape asks Harry, in his first-ever Potions lesson, where he would go to find a bezoar.

The bezoar appears a second time in Harry Potter and the Goblet of Fire when it is the "key ingredient" that Harry forgets to add to his antidote in Potions class because of his anxiety over preparing to ask Cho Chang to the Yule Ball.

Bezoars are not mentioned again until Harry Potter and the Half-Blood Prince. In Professor Slughorn's Potions class, Harry is called upon to produce an antidote. Rather than creating a complex potion that would counter the mix of poisons in question, he simply retrieves a bezoar from the supplies cabinet. Professor Slughorn absently tucks it into his bag.

The bezoar then becomes vital to the story as Ron is given some poisoned mead while visiting Professor Slughorn; while Slughorn panics, Harry recalls the bezoar, retrieves it from Slughorn's bag, and feeds it to Ron, thus saving Ron's life.

Analysis
Like Boomslangs and Dittany, bezoars exist in the world outside the series. In our world, bezoars are balls of compacted hair that collect in the stomach of a person or animal, and were once believed to have magical curative powers, very similar to what they are given in this series.

With bezoars being so very useful in the case of poisoning, it is rather surprising just how uncommon they are. It seems that there are only a few, even in the supply cabinet in the Potions classroom; and it seems that Professor Slughorn, aware as he is of the effects of potions, does not routinely carry one in his bag. We must assume that they form very rarely in the wild.

Because it is mentioned that it is an ingredient in the antidote Harry is making just before Christmas in the fourth book, and because it is used directly, bezoars are listed both as magical devices and potion ingredients on the Magic overview page of this work.

Questions

 * 1) When was the bezoar first mentioned?