Marine biology rarely makes the news. Sea cucumbers even less so. This is no exception.
Sometime in the mid-1870s Swedish zoologist Johan Hjalmar Théel dredged up the ruined wreckage of a deep-sea sea cucumber. It wasn't very impressive, mostly because dredging up creatures without internal skeletons and tough skin tends to pulverize them, but he went ahead and classified it as Enypniastes eximia. While sea cucumbers often aren't gifted with common names, Enypniastes eximia has had a few. The first of these, granted by the comparatively sane and sober biologists of yore, was simply 'deep sea Spanish dancer'.
This was problematic, as the Spanish dancer is a completely unrelated beast, not even in the same phylum (both creatures do belong to Animalia, so not completely unrelated). The Spanish dancer is a dorid nudibranch, AKA a sea slug. It does indeed look like a slug when on the ocean floor, but when swimming it is a bright red or pink undulating sheet with ruffles and speckles resembling white lace, and it dances. It's pretty cool, and it does bear some resemblance to Enypniastes eximia.
However, Enypniastes eximia also, in addition to looking like a Spanish dancer, also looks like a headless chicken. There was a brief period where the hypnotic, undulating swimming of the cucumber earned it the nickname The Dreamer, but you have to admit, Headless Chicken Monster is catchier.
The Headless Chicken looks pretty much like a human heart with a ring of tentacles for a head has decided to go for a swim. It has airy and graceful 'fins' that are more fitting to a jellyfish than a real fish, and it has a fringe of enlarged papillae just below the 'neck' that can, at certain angles, look like stubby, plucked chicken wings. For aficionados of sea cucumbers, these are closely related to 'sea pigs' (the Scotoplanes), but are a bit darker pink/red and with the 'legs' removed and replaced with fins. As you might expect, the Headless Chicken prefers swimming to crawling on the bottom, but does drift down to the sea floor to feed at times.
The terms Headless Chicken Monster and Headless Chicken Fish are commonly used when remotely-operated submersible vehicles (i.e., unmanned submarines) come across Enypniastes eximia, but the term is colorful enough that you will see blog posts and pictures floating around the internet as well. You can check out photos here and here.
Kingdom: Animalia (Animals)
Phylum: Echinodermata (sea stars and their squishier relatives)
Class: Holothuroidea (sea cucumbers)
Order: Elasipodida (spiky and frilly sea cucumbers)
Family: Pelagothuriidae (swimming and frilly sea cucumbers)