In Dan Machi on the side, Aiz faces off against a floor boss (Udaeus) all by herself (with 1 observer). Yet, it seems apparent that everyone knows the monster's name.
How would the adventurers know the floor boss's names? In this case, the particular floor boos doesn't make any verbal sound for some reason. Presumably, other floor bosses will not speak but may make noises.
Based on Danmachi's lore, it is possible (i'd like to say almost sure but no proofs here) that monsters have something to do with gods. If it is the case, maybe gods were those who named the floor bosses.
There also are more possibilities. Those monsters, unlike Grimgar's, are more likely to respawn like basic RPG monters. If adventurers already fought a floor boss, it is legit it is has a name. Maybe those who are the first to defeat a kind of boss are given the great honor to name it.
Sadly, Danmachi's doesn't tell us much about it so it is only speculation. Maybe we'll learn more about it later in the story.
Hope that helped.
