Nunca é explicado.
Em The Treason of Isengard , Christopher Tolkien sugere que era provavelmente um Nazgûl, embora ele admita a contradição:
The dark shape 'like a cloud yet not a cloud' that momentarily cut off the moon's light is surely reminiscent of the shadow that passed over the stars as the Company journeyed on from Hollin in 'The Ring Goes South' (VI.421 - 2), and which Gandalf unconvincingly suggested might be no more than a wisp of cloud. Then too Frodo shivered, as here he 'felt a sudden chill'. As I noted (VI.434), the former incident was retained in [the final draft of Fellowship] but never explained: the Winged Nazgûl had not yet crossed the Anduin. But it seems likely to me that the shadow that passed across the stars near Hollin was in fact the first precocious appearance of a Winged Nazgûl.
History of Middle-earth VII The Treason of Isengard Chapter XVII: "The Great River"
Claro, isso é obviamente incompatível com a declaração de Grishnákh. Infelizmente não há outra explicação imediatamente óbvia; Nenhum dos rascunhos ou cartas de Tolkien entra em mais detalhes sobre isso.
O primeiro rascunho do capítulo é notavelmente semelhante ao final:
Frodo kept looking up at the sky, partly because of its beauty, partly because of Elrond's words. Suddenly he saw or felt a shadow pass over the stars - as if they faded and flashed out again. He shivered.
'Did you see anything?' he said to Gandalf, who was just in front.
'No, but I felt it, whatever it was,' said the wizard. 'It may be nothing, just a wisp of thin cloud.' It did not sound as if he thought much of his own explanation.
History of Middle-earth VI The Return of the Shadow Part 4: "The Story Continued" Chapter XXIV: "The Ring Goes South"
E isso não é alterado drasticamente nas revisões posteriores. Nós não sabemos o que Tolkien pretendia que isso fosse; se foi um erro que ele errou ou esqueceu de corrigir, ou se foi um mistério intencional. Nós simplesmente não sabemos.
A explicação mais plausível no universo é jamesqf , a partir de um comentário sobre a questão:
More likely that it's general orders for the Nazgul to stay east of the river, but one got sent on a special secret mission, especially as Sauron likely doesn't want the troops to know about the Ring &c.