Rathdown is very correct. The sporran is, if nothing else, a pouch. People have been wearing pouches since time immemorial as a way of keeping their important possessions close at hand. One can perhaps research certain styles of sporrans and be able to say "this style first made an appearance in the eighteenth century," or some such. But as to the sporran itself, it's been around for much longer than the kilt.

In the twelfth century, the French Benedictine historian and chronicler, Guibert of Nogent, was describing in a text a vision he had of devils dancing about, and he described them as wearing their pouches "in the manner of the Scots, hanging forward from their haunches, as they are wont..."

I found this particular reference fascinating when I came across it because it predates the kilt by more than 400 years. But apparantly even then the Scottish people had the habit of wearing their purses hanging forward off their hips, as the sporran is typically worn with the kilt. Moreover, they were well enough known for this style that a Benedictine monk writing in France could make reference to wearing a pouch "in the manner of the Scots" and expect his readers to know what he meant.