Latest chapter 764 of One Piece revealed that the Celestial Dragons called the D's as 'Devils' & they are the natural enemy of the Gods.Celestial Dragons are self proclaimed creators of world, i.e, Gods.

Ok we all know this now , lets go ahead & observe the word 'Devil'.Doesn't the word ring a bell , yeah the word has appeared countless times before us , 'Devil Fruits' !

So now let me summarize my theory.

    A long time ago ,in Ancient Kingdom ,people of D clan created fruits .Theses fruits after eating gave that being supernatural powers.We all know what kind of powers it gave,so no need to go in detail.
    So how did ,they create devil fruits.This has many possibilities :

1)They defeated a monster of sea ,and divided it's power to create fruits.
2)They created tree/trees which yielded a fruit at regular period.
3)They created fruits by experimenting similar to the Caesar & Doflamingo's smiley Dfs.

    But I like the 2nd possibilty the most.So I'll hang onto that the D peolpe created tress which yielded fruits.
    Eventually ,a war broke out between Ancient Kingdom & 20 allied nations.Ancient Kingdom lost.
    But the so people of Ancient kingdoms were astonished that these D peolpe had supernatural powers(those who had consumed the fruit).
    Ancient Kingdom in order to avoid the fruits falling in hands of the hands of 20 allied nations(world govt), distributed the trees across the grand line ,the most dangerous place in whole world.
    Eventually people who roamed the grand line started finding those fruits and gaining powers.So it didn't took much time for the fruits in coming to spotlight.These tenryubito's who feared the power of these fruits & the people of D clan, started calling the people with D's as 'Devil' and their created fruits as 'Devil Fruits'.
    It could explain why Devil fruits are difficult to find because they were hidden by the people of Ancient Kingdoms.

So here is what I think,that D's are creator of Devil fruits.