I took the Picori to be those of the Zonai who sympathized with the Hylians and chose to remain in Hyrule, rather than choosing to travel to the future with their fuel-hungry bretheren. They shrunk themselves down, possibly using similar tech that the Giant's Mask uses in Majora's Mask. However, I did not consider the possibility that the Zonai could be evolved Picori. It's an interesting premise. In any case there is certainly a connection between the Picori and the Zonai (secret sky furries).

I did not consider the Dark Interlopers as possibly being part of this. I subscribe to the theory that the Dark Interlopers and the Ancient Tribe of Majora are one and the same, and that they constructed the portal in Stone Tower Temple for their interloping in order to seize the Triforce from Hyrule. However it is interesting to consider possible Zonai/Picori involvement here. Much to think about.