From Tolkien Gateway
Usage of plural
- Drúin is the simple plural of the word.
- The collective Drúath was modified as early as the First Age when it became known that they were enemies of the orcs. Later used to refer to a large number of the Drúedain.
- Drúedain refers to the race as a whole.
Since Elvish languages don't have gh it was derived as drughu > dru'u > druu > drû