Originally Posted by
Korsha
As far as I got from the Chronicles lore books (my understanding, don't take this as absolute truth), most, non titan-forged, mortal creatures seem to have evolved from elementals. Seems like with enough exposure to the spirit of life, over enough time disconnected from respective element energy, they become fleshy. On Draenor the clear examples are the Collosal-Orc Breaker line, were we see each "generation" become more fleshy and more 'human'.
On Azeroth, we see pretty much every elemental plane having their own version of dragons, if those were common in primordial Azeroth, I guess that's why there were dragons of so many colors and different breath attacks. One race evolved from many elemental versions. I also like to think of Gilblins as, rather than being a gimmick, is a hint that the Goblin race evolved from a type of water elemental.
As for trolls, they don't really seem close to any animal, so I would assume, as someone already said, the Zandalari scales are from millenia of dinomancy. I can easily see rituals of blood drinking or heart eating, or even simple totem ornaments, to help warriors embody the strength of the Ravasaur or the cunning of the raptor.