If you have sufficient prestige, a new dynasty will be generated from one of your noble families (they will however have one of the pre-set dynasty names from your culture group)
If your prestige is low, you'll get a noble from a nation with which you have a royal marriage.
Needless to say, i always choose my royal marriage partners carefully to avoid getting an Askanier.
In fact in my last game as Brunswick i switched to Noble Republic to get him off the throne, elected a different guy (the names here can also actually come from the Admiral/General list). Then switch back to Monarchy and your former republican leader is now a King. That is one way of generating dynasties that dont actually exist in the game and would otherwise be impossible to obtain.