My guess? Something left over from the Dawn, perhaps constructed by the Nords themselves but forgotten. The connection of bird/air/sky with metal/ground/forge is interesting, speaks to Kyne as war-wive of Shor.

Vivec.

Once, as the sun beat from its high-home, Jonder the Tiny, the one who ran ahead, came over the hill to tell what was seen. Amidst a vast plain his eyes had met a monument of a bird, whose eyes and beak were opened in flame. When his brothers and sisters crested the hill, they too saw its glory, but they were afraid for no elven settlement could be seen to the horizon.

"But this is not seemly," said Kluwe, who went by Loate when hiding his face. "Is not this wide land fit for harvest? Why have not the elves, vile to their core, seen to exploit and tame it?" They asked of their elven captives (for they had many) what they found unfit about these plains. Yet even the captives who still bore their tongues could say nothing of the valley. They looked with fear at the winged colossus, and from their babblings did the warriors of the Jorrvaskr learn that it was older than even the elves themselves. Of those who wrought it solid from its mother-stone, nothing could be said, but it was known to drive a magic almost as old as Nirn itself, some remnant of the gods' efforts to render a paradise in Mundus before the shattering of Lorkhan.

some remnant of the gods' efforts to render a paradise in Mundus before the shattering of Lorkhan.

Redguards have memories-in-stone. Nords have memories in fire. Kodlak was burned in it, and his essence powered the Skyforge, allowing for the creation of ancient Nord weapons and armor, lost lore. It could be a memory forge.

Seems to be as clear an answer on the subject as we are likely to get.

Heh, you aren't letting go of the subject of memory stones, are you? Cool, I like it.

Maybe it installs memories to the blacksmith who's using the forge? a journal of memories which subconciously tells the secret of making their armour?
Well at least it'd be a convenient way of getting a large order of armour done with a bunch of untrained novices in smithing.

Linked to the dragon ruled era would be the most obvious answer. It makes ancient nord armor, draugr weapons and is reawakened by souls. I could see dragon priests sacrificing servants upon dragonfire forges to empower words of kyne (thu'um) enchantments - thus a skyforge. The thu'um enchantment would wake up once it has been charged. Although I do not know in all high hell this enchantment's effect would have. Maybe it installs memories to the blacksmith who's using the forge? a journal of memories which subconciously tells the secret of making their armour? Well at least it'd be a convenient way of getting a large order of armour done with a bunch of untrained novices in smithing. So yes by all this I mean it was built by the nords who worshipped dragons and was heated by dragon fire. And eh this is just some speculation to throw down this way, eh.

A gift from Zenithar.

A too-silly thought ripple that left an impression in the stone-like-cartilage of the foetal Kalpa. This one in particular: A typical yet strong Dreughish daydream. So great does the feather-envy bubble like rude gasses in the chalcum-ichor of that folk, that besides alotting daily time for fish-suppers and for egg-laying, the Dreugh have prescribed hours for peeking their fore-carapaces above the surface (at the great risk of dessication of the eye-clusters) to gaze up that the vault of Kynareth-All-Around-Us in stultifying cloud-lust, yearning for the chance to reign amongst the airy peaks in metal palaces, eating fire and then singing conflagrations on the wicked leg-swimmers below (for such is how the Dreugh imagine the typical Avian career).

So how does that relate to the Skyforge again? "A Dreugh did it" might be the Lore Forum's new motto. That or, "It happened in a previous kalpa." or maybe just, "Um... CHIM?"

"A dreugh from a previous kalpa did it with CHIM."

...I like that more than it probably deserves.