Oarfish (Regalecus glesne) are one of the world's longest fish reaching 17m.This one was estimated between 5m and 10m in length. I find it interesting that they apparently swim backwards, in a vertical orientation. So, really, they swim down, tail first.
Their strange appearance may have provided the basis for the sea serpent myths told by early ocean travellers.
Not only are they elongated, they also have a prominent dorsal fin which gives it an unusual "serpent" appearance.
Very cool stuff.