The custom of wearing one can be dated to at least the thirteenth century by which time we see it depicted in sacred art. Typically when we think of the zucchetto, we tend think of it being a small circular disc, such as this one, seen in the window of the Roman atelier, Gammarelli:
But as with so many things of this sort in the life of the Church, if you're looking for there to be utterly consistent uniformity always and everywhere, that isn't usually to be found to be the case. In the case of Pope Pius VI (who sat on the Chair of St. Peter from 1775 to 1799) we find a zucchetto with a rather different form and appearance, still white as is appropriate for a pope, and still sharing many of the basic characteristics of a skullcap, but also containing an embroidered, decorative Greek key pattern at its base and being larger in size the many modern zucchetto:
As you can see, it has an almost biretta like appearance, though it is obviously not a biretta and soft and malleable so as to be able to be worn on the head.
In terms of how it might have appeared when worn by Pius VI, I suspect it would have looked something akin to the skullcap traditionally worn by the Capuchins. Namely, a skull cap that more amply covers the top and back of the head:
Just another interesting tidbit of history for those of our readers with a particular interest in prelatial vesture.
