I don't suppose that tunings are the kind of things to which one can ascribe an original owner or inventor, though some people certainly have their ideas. For some, the first proponent of drop D tuning would have been John Dowland, a Renaissance composer between the 1500s and 1600s who composed music for the lute which, when arranged on the classical guitar, required said guitar to be tuned using drop D. This sounds a little far fetched to my ears. We can at least be sure of some of the early proponents of the tuning in the rock mainstream. Acts like the Beatles and Led Zeppelin no doubt helped to popularise the tuning with their heavy and anthemic hits 'I Want You (She's So Heavy)' and 'Moby Dick' respectively.