Anabaptists did exist prior to the Reformation as there were always those who rebaptized. Anabaptists were not a denomination per se. Their belief systems varied widely. You will find believers who rebaptized all of the way back to the Roman period. They may not have been called Anabaptists. I'm sure that you have read the quote by Stanislaus Hosius? I'm not sure of its context, so I'm not hanging my hat on that particular quote. However, Baptistic ideas (as in modern Baptists) do have kindred spirit all through the history of the church.
The problem is in tracing these various groups because they were decentralized in their church government and the names they were known by were picked by the dominant state religion of the time. However, be that as it may, they existed.
What I am looking for with wmFights is a historical theology resembling the thinking of the Reformation, especially on the four Solas. Rebaptism is not a foundational Reformed teaching, is it? Of course there have been various heretical groups all along.
It is wrong to confuse anapabtists with donatists though. The latter rebaptized for a specific reason of the initial baptism thought invalid because of the apostacy of the priest. It was not a reflection of any theology of baptism being an expression of internal rebirth through adult faith, which we find in (ana)baptists.