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The theory of Boldogs as Orc-shaped Maiar entered the legendarium at a relatively late state, and was not taken up in the published Silmarillion.
Some of these things may have been delusions and phantoms but some were no doubt shapes taken by the servants of Melkor, mocking and degrading the very forms of the children. For Melkor had in his service great numbers of Maiar, who had the power, as their Master, of taking visible and tangible shape in Arda. ('Morgoth's Ring', "Myths transformed", text X')
Boldog (…) is a name that occurs many times in the tales of the War. But it is possible that Boldog was not a personal name, and either a title, or else the name of a kind of creature: the Orc-formed Maiar, only less formidable than the Balrogs (Author's footnote to the text X)
Melkor had corrupted many spirits — some great as Sauron, or less as Balrogs. The least could have been primitive Orcs. (Author's note to text)