When I first used the DM I saw that there were only available the enemies I had encountered during the original campaign. But then I played a couple of user-created modules and saw some monsters that weren't available for me to add in DM. However, after some runs in fan dungeons, some of those monsters became available to me as well... I assumed you first had to collect enough info about them (4/4 in the bestiary) in your runs in order to be able to use them in your own dungeons. But then this is just an assumption. I for instance could use Minotaurs and Winter Wolves from the beginning, and I am not using the mod that allows accessing to all the locked DM content.