User blog comment:Tatmtn/Huntsmen and Huntresses/@comment-25405817-20141017222624/@comment-3146930-20141017225343

If I had to venture a guess, I would say that Hunters came about some time after the initial war with the Grimm. Retired soldiers with high levels of skill couldn't give up fighting and so would seek work "hunting" Grimm for fame, glory, and maybe some pay. Eventually groups of first-generation Hunters got together to fight larger prey. Once finally reaching an age where they could no longer fight on the front line, they may have started teaching others their skills to save and help others. Eventually an official combat school was formed and the rest is history.

Of course this is only one possible theory.