A warrior-queen of Lancre whose precise story is lost in the mists of time.1
In a famous portrait, she is depicted as looking her foe full in the eye with a "Make My Day!" attitude, a gesture the foe cannot reciprocate as he has been reduced to a severed head hanging from her right hand. She is wearing full armour, including an impractical winged helmet and a breastplate created for somebody...erm... more full-figured than Magrat Garlick.
She owned a war-chariot festooned with hooks and pointy bits, including the two mandatory scythes on the hubcaps. Her favourite horse was called Spike.
When beset by Elves, Magrat rediscovered Ynci's armour in the castle armoury. Just as a pointy hat confers witchiness on the wearer, the winged helmet instilled an atypical warlike spirit in her (although she was disappointed that the breastplate left a lot of empty space between itself and Magrat). Several Elves then discovered to their cost they had effectively cornered a mongoose who viewed them as squirming reptiles. Even Greebo decided that fighting Elves was preferable to crossing Magrat, and a cat's instinct for trouble is usually acute.
Lancre has had Warrior-Queens before; it has had Witch-Queens before; Magrat's distinction is in becoming its first Warrior-Witch Queen. And the irony is, if Nanny Ogg is to be believed, that Ynci may never have existed at all; she may have been dreamed up by an amateur historian who was disappointed that Lancre appeared to lack the right sort of history.
But Headology begins with the headwear....
(1)Okay, not so much lost in the mists of time as never existed. She was made up by a later king of Lancre who felt a little inadequate in the area of historical symbols to rally the populace. He had a smith make up some suitably spectacular armour.
The first reference is to Boudicea, to whom a Victorian historian added the scythes on the hubcap of her chariot. Such optional accessories are not mentioned at all in Roman histories, and as a race who had a lot of pacifying to do in Celtic regions, they would have surely mentioned it if their enemies' chariots held an extra hazard of this nature, coming at them at roughly knee-height... So maybe that Victorian historian considered history needed to be improved on, and added a detail from his own imagination.
Or did he? Irish folklore preserves the legend of Cuchulain, the great Warrior of Ulster, whose personal chariot was festooned with hooks, barbs and spikes on all its surfaces, as was the armour on the horses drawing it, so as to better rend and impale his foes. The Tain Bo Cuilaghne (a history of a war between Ulster and the rest of Ireland, indicating there's nothing new there), also tells us this chariot had the obligatory scythes on the hubcaps... so maybe the Victorians had read the Irish myth and had transplanted it to Celtic Norfolk...
A King making up bits of history is probably a definite link to Edward I who created a Round Table that might have been King Arthurs. But made up histories could be a link to Geoffrey of Monmonth's History of the Kings of Britain (which was also made up).