I should specify - I mean for full frame sensors like the 6D.
The Rokinon/Samyang 8mm is designed for crop sensors - I have one for my Pentax and I see that theyve recently made a 12mm to suit full frame sensors.
The sigma 8-16 is a great lens but its rectilinear so not a fisheye (as well as being for the APS-C sensors)
I guess you are going to learn something new today. Rokinon 8mm works just fine and attaches just fine to full frame cameras. But it isn't 8... it is more like 8.5, so the complete circle image isn't, rather it is semi-circular on the body. I prefered how it looked on my crop sensor 60D, but it does work on a full frame.
As for the sigma 8-16... turns out you are right. I just started... and I could have sworn someone I had a conversation with raved about their sigma fisheye... maybe it was a prime they were talking about.