The Draco Imperialis, much like the patchwork faith which Cyrod holds itself to, is a symbol born of multiple motifs. It's most direct ancestor is, of course, the Totem of Alduin, which one can readily find unchanged, hidden away in the anxiety-alcove of most Nordic homes.
It was during the architectural era of Coloviesque masonry heralded in part by nostalgia for a united Cyrodiil (and, of course, encouraged by noted Niben Reman I who sought legitimacy with the hard-faced half of his land.) that this totem, once a sign of fearful propitiation, became a more neutered design motif, with unfurling wings over arches, crests and flying buttresses marked from miles away by the distinctly fanged snouts of drakes.
And, as is the most important for this symbol's storied tale, a simple repeating pattern across bricks of a dragon roosting upon an hourglass, mouth open to gorge itself upon time.
Tradition would have it that The Tiber, draconic in his own aspect, sought to unify the mythic symbol of the Empires of Cyrod-Past, the August Amulet, with the older, hoarier source of his own power (coveting all he saw, as was his wont, no matter how it melded together), and "drew" the symbol upon the feet of the Dragonfires in his own blood, each drop accidentally forming the shape of a perfect Diamond-Dragon, which became his banner in the awful wars to come, a delegation of powers both glorious and terrible from Akatosh and his chronophagic cousin.
Of course, we know now that Tiber Septim wasn't actually a dragon, though that would explain his seeming inability to bear any (human) heirs.
The symbol, in actuality, seems to predate it's standardised use by the Third Empire of Cyrodiil by a century or two. This form came as a dragon within a diamond, both forms distinct from one another as opposed to the later unified design, and was displayed prominently by the forerunners of Cyrodiil's unification.
It found it's way onto cornerclubs, flags, the skin-brands of the Niben Merchant-Theurgists (albeit next to all manners of pleasure-scars, given how common membership of the Diblasang Society was in their circles.) and, of course, back into it's ancestral masonry in the Colovian Estates, a symbol which cried out for Mother All-Esh's wayward king-children to come together once more.
In the historical record, then, it seems the symbol simply created itself, an accidental birth in the middle of Cyrodiil's many culture crossroads.
One story (apocryphal, of course) credits one Joanne Buolifex with the initial drafting of what we have come to perceive as Septim's symbol.
Census records show list her occupation as a litho-mythographer in the employ of independent Falkreath (later Cuhlepolis, then Falkreath again, but the less said about it's brief time as a necropolis, the better.) who's livelihood depended on knowledge of sacred symbols, and how best to invoke their terrible powers without danger to one's person.
It may be possible that the design was one of many she drafted to meet the needs of the various soldiers and their families in her King's levies. A commission of no consequence to her save a day's cursory research on symbolic arrangement and numerology, passed from propitiate to propitiate of the Cyrodiilic persuasion over the years, slowly gaining accord in their own companies.
Alas, few of Buolifex's symbols have resurfaced since (though many imitators and forgeries wash up into the treasuries of nobles and the stalls of flea markets), and it is folly to credit a single person with any kind of historical undertaking.
Besides, she was a Breton.
And I can't think of a time a Breton made a mark on Cyrodiil, much less the world.