Det förklaras aldrig.
I Treason of Isengard föreslår Christopher Tolkien att det förmodligen var en Nazgûl, även om han medger motsägelsen:
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"
Det här är naturligtvis oförenligt med Grishnákhs uttalande. Tyvärr finns det ingen annan omedelbar uppenbar förklaring; ingen av Tolkiens utkast eller bokstäver går in i mer detalj på den.
Det första utkastet till kapitlet är anmärkningsvärt som det slutliga:
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"
Och det ändras inte drastiskt i senare revisioner. Vi vet inte vad Tolkien menade för detta att vara; om det var ett fel han missade eller glömde att rätta till, eller om det var ett avsiktligt mysterium. Vi vet bara inte.
Den mest troliga förklaringen i universum är jamesqf , från en kommentar till frågan:
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.