This great video from National Geographic shows three rearely seen sharks on video, deep down in the ocean.
The Six-Gill Shark is special because most sharks have five gills. Apparently, according to the video, these sharks date back to pre-dinosaur times. This remanent is still going strong. They can grow up to 15 freet long, and can dive as deep as 6000 feet.
The Chimera or Ghost Shark is related to both sharks and rays, and could be one of the oldest groups of fish known. There's a video on Youtube showing one with it's egg-sacks out, which is quite astonishing.
The Pacific Sleeper Shark can grow up to 25 feet long. Their flesh has a toxin in it that produces symptoms of drunkeness in the humans that eat it. They feed mainly on Giant Squid and Octopus.
The Dumbo Octopus
This cutey is called the Dumbo Octopus, so named because of its fins which look like the ears of the Walt Disney character. They generally live at depths of 3000 - 4000 meters, but have been sighted at 7000 meters. At about 20cm, they are not that big, but strangely prefer to swallow their prey whole. There are about 37 species in every ocean.
Here is a link to the BBC Website page for the Dumbo Octopus for more info.