There's Ivailo of Bulgaria, the capable peasant leader who became Tsar for a while in the late 13th century. Didn't manage to get a dynasty going, unfortunately.
For a non-medieval example, there's the Serbian uprising in the early 1800s, which crowned its leader, Black George, and started off the Karadjordjevic dynasty. A later led to the creation of a rival dynasty, also of peasant origins.
Interestingly, Ivailo was apparently a swineherd, and Black George a swine merchant. Maybe there's something about dealing with the sometimes extremely cunning pigs (instead of, say, the monumentally stupid sheep) that prepares a man for good leadership.
I've always wondered what would happen if Gyorgy Dozsa's rebellion succeeded. The demographic and economic consequences along would be massive - maybe even massively good. A Hungarian crusader-kingdom with a defanged, purged and partially renewed aristocracy and fairly high level of peasant rights? Maybe it could even successfully resist the Ottoman invasion?