The description of the Synad from Complete Psionic actually leads me to theorize that the Synads are the descendants of Tiraphegs, a really really weird (and obscure) monster from the original 1st edition Fiend Folio, which had three heads (two of them featureless like a Synad), three legs and three arms...
I don't think the Tirapheg has ever since then been mentioned. Though Spelljammer back in 2nd edition would speak of an ancient race that ruled much of the Material Plane, called the Juna (also the possibly rumoured Zookeepers) who were described as Tripartite beings.
Anyways my idea is that the Juna were first, their descendants were the Tiraphegs, and now from somewhere across the Shadow Plane, from some distant world hidden somewhere the Synads have arrived.
Anyone have their own theories on this?
I think I need to find out more about what the heck a tirapheg or snyad is before I have any thoughts about them. I only recognized "juno."