Fisheyes and ultra-wides are two totally different animals.
For ultra-wide, you have a lot of choices... both zoom and prime, that will work on an FX. Tokina's new 15-30, Siggy 12-24 (which I have) and others fit the bill. Although the ALL display some sort of distortion (barrell, pincushion or compound), they are considered 'rectiliner' lenses as they do their best to keep straight lines in the scene straight.
Fisheyes, however, do not make any attempt be rectilinear. The further a straight line is from the optical centerline of the optics, the more it is curved. And there's two types of fisheyes... Full-frame (not to be confused with full-frame sensors), and circular. Full-frame fisheyes provide a 180° FOV from corner-to-corner, while circulars have a 180° FOV across the short dimension of the sensor. You end up with a circular image on a black field.