I think that in general loot from these dragon bosses is quite good, everytime you kill them you can loot some expensive creature products, which you can sell to Yasir.
There are plenty of rumors that you can loot some expensive and very rare items. I think that it's even hard to call it "rumors" since this info you can find among an official fansites (so it is a trustful source), but we have to remember that fansites are built by players, not by Cipsoft. Sometimes this info are simply wrong. For now the only confirmed rare item is Maxilla Maximus which you can loot from Zorvorax, dragon boss from Edron.
Maybe in few years a very lucky player will publish some screenshots with his unknown for now rares looted from these bosses, who knows! As soon as it happen, I'll edit my answer :)