I do have a Zenitar - bought it ten years ago for my *ist DL - but the lack of an "A" setting makes it quite a bit more cumbersome in actual practice - plus all the rubber on the focus ring fell off when I went to use it and I can't find most of it to glue back on.
I suspect the optical quality will definitely not be able to compete with the Samyang. I do still have my 10-17mm which is not too bad on the K-1 but it's for sale (see Marketplace!
) but again, the sharpness is pretty good but the purple fringing is really bad. The Samyang 12mm seems to have very, very good optics and you really need to be looking hard to find fringing.
The other thing is the aperture blades - virtually all the fisheyes for Pentax have six blades, which is IMHO not great for "starbursts" - I want at least 7 (giving you 14 points) or higher - I think that helps a lot in nighttime fisheye shots. I think your only options are the Samyang 12mm or the Sigma 15mm. I have wanted a weathersealed fisheye for a long time and assume the FF fisheye zoom that's on the lens roadmap would be sealed, and hopefully the optics will be much improved over the 10-17mm, so the only question will be the blades. If it has 7+, it will be a dream lens for me and the Samyang will go up for sale. The zoom may be a little slower, too, like F3.5 vs F2.8, but I rarely shoot fisheyes wide open, and if I have to bump up the ISO a little more, that's really not that big of a deal on the K-1.
I haven't played with the lens correction settings in Lightroom, maybe I can add a little center fishiness that way.
There actually is a profile in there for the Samyang 12mm, and it does make straight lines across the image, but the picture is so wide that things are really stretched. It's a very strange look.