It's a Mesoplodon Pacificus

(TOKYO - Japan) - The  researchers of the Kagoshima City Aquarium and of the National Science Museum has examinated the cetacean beached in the Japan's southern coast five month ago. The spokesman few days ago said that it was a 21 feet female of Mesoplodon Pacificus (Longman's Beached Whale) -ziphiidae family-. 

It's an extremely rare species (has never been identified a living specimen) also found in Australia, Somalia and South Africa but in those cases were mostly skeletal or young; so this one is the first adult found.

Those cetaceans known also as indo-pacific beached whale have a long beak-shaped mounths and resemble elongated dolphins but it's bigger, untill 6 meters.