I believe Jurgen windcaller was a dragonborn. As evident in the dragonborn dlc when miraak dies we absorb his soul because dragonborns have dragon souls. So we know a dragonborn can absorb another dragonborn's soul. Well if you go to Jurgen windcaller's tomb for a second time after getting the horn of Jurgen Windcaller you will absorb a dragon's soul from the tomb. This leads me to believe that Jurgen windcaller is dragonborn.