One of the rarest is the fish formerly known as Bananogmius, now Pentanogmius. The name change is sad, really, because I quite enjoy saying "Bananogmius", although not quite as much as I love getting other people to try saying it. It was a large fish with no big teeth, yet a large sail fin running the length of the animal. It had only tiny pectoral and pelvic fins.
The RMDRC has a new articulated specimen (photo above) recovered this year off of private land. The skull is to the left, with the sail fin taking most of the upper half of the large jacket. This one is much larger (over 5 feet long), more complete, and better preserved than the new Denver specimen. It has been "show prepped" to a plaque stage, and will be molded as is. This will get the fish ready for 3d reconstruction using casts of the original bones.