Oarfish can grow up to 10 metres long and are the basis for the stories of sea serpents told by ancient mariners.
It is very unusual to see one in the UK's seas, which are quite shallow.
They normally stay in the 1,000m depths of the Atlantic Ocean.
Not a happy ending
London's Natural History Museum had been hoping to get the oarfish to preserve it.
But unfortunately it's already been cut up into fish steaks!