Boggan was created by Lady Eiluned; the story is as follows:
Originally Boggan was merely a chimera, a servant of Lord Dougal. One day, in the middle of polishing one of its master's urns, it is driven out of the hall by Pooka (who is playing his usual tricks). Boggan then wanders about Arcadia, asking if the urn belongs to whomever it meets, so that it can return it. For Boggan, as a simple cleaning chimera, is not able to recognise people.
Everyone Boggan meets says that the urn is not theirs, until it comes to Lady Eiluned. She asks if she might inspect the urn, at which point Boggan hands it over, and she says that it is her urn (on the principle, perhaps, of 'finders keepers'. Boggan asks her (thinking her Lord Dougal) for true sight, so that it will be better able to serve in future. She agrees to this, but requires that it perform a service. Boggan agrees, and she grants it the ability to recognise those around it. Immediately Boggan sees that he has given the urn to the wrong person!
However, Boggan is faithful and honest, and sees only that it has been foolish. It asks the lady what service it must perform, and is told to present itself at the Halls of the Mountain King, there to serve the Trolls. Boggan does this, and the Trolls set it to work. But Boggan knew only the service of Lord Dougal, and angers the Trolls by its attempts to treat them as Sidhe.
When Lady Eiluned comes to collect Boggan, for its service is over, she refuses the Troll's offer of payment, but they insist that honour makes it necessary. So she takes the dullest gem from the treasure chamber, the one which no amount of Boggan's cleaning had made shine. Had the Trolls known that this was the Emerald that Lies at the Heart of the World, they would never have given it away, but the Mountain King was dead and so the gem was lifeless and unrecognised by all but Lady Eiluned. But she also pledges more significant service than the use of a chimera, to restore them to greatness by bringing back their leader.
When Boggan and Lady Eiluned had left the Halls, Boggan asks her for another gift. For its heart is heavy that it had angered the Trolls, and it wants the gift of empathy, to know another's needs. She agrees to grant this gift, if Boggan would perform another task: to get a single scale from a Dragon, given willingly. Boggan agrees to attempt the task.
[Boggan gets the scale, but in so doing realises the limitation of its last gift: while it can tell roughly what pleases the Dragon, it sees also that it is impossible to provide anything pleasing for it (Dragons are odd like that).]
After giving the scale to Lady Eiluned, Boggan asks for a third gift. Only this time it doesn't ask for something it thought it wanted, but rather for what Lady Eiluned values most. She agrees, and sets a final task: to bring her an urn crafted by Lord Dougal. To Boggan, this seems impossible, harder even than getting the scale. Lord Dougal would certainly not give away an urn to a creature that had already lost one, and it could not steal or cheat. But Lady Eiluned takes pity on it, and gives it the urn Boggan had earlier given her. Delighted, Boggan then gives it back to her, and she grants his wish.
[She puts Boggan in the urn and carries him through the Gate of Horn, with herself protected by the dragon scale, and using the emerald to open the way (for the gates are one way). She then reappears through the gate, only now she is brighter than the sun and from this point on no longer dwells in Arcadia or has much to do with other fae. Then Boggan, now with a fae soul, is born through the Gate of Ivory. The urn is lost and the emerald is not heard of again.]