here is my idea and i know this is a huge stretch, but hear me and out and try not to be too critical. i think that the picori somehow become the kokori. the picori are never seen or heard of in any other game besides for MC, and in MC the picori that lived in the forest were the protectors of the forest. what if all the other picori (the mountain and town picori) started to die out and a magical being (the deku tree) was somehow present in the forest a long time after MC. this magical being decided to help the remaining picori that were on the verge of extinction and he turned them into the kokori so that they could survive and protect the forest as relatively normal humans (that just dont age). i know its a stretch but we have no idea how long OoT comes after MC (if you believe in that timeline) and it would explain why the picori arent around anymore or ever spoken of anymore. also in the wind waker, the deku tree (who is the child of the original deku tree) states the koroks used to have different forms before they became woodlike creatures. one of the sages in WW says that he used to be a kokori and one of his descendants is a korok. so this is pretty much saying that the koroks used to be the kokori. if they kokori could change forms into the koroks, it wouldnt be a huge stretch to say that the kokori could have also been something before they were kokori.
picori changed to kokori by deku tree, kokori were changed to koroks by the son of the deku tree (who is still called the deku tree).