The name of this song is called Mimikyu no uta.
Though this new Pokemon may look like a horrible looking and fake Pikachu, it is actually rather sad. After listening to its song, I felt sorry for this lonely pokemon!
This awesome Japanese song was officially released on Oct 20 2016 by the Pokemon co. ,as a new introduction trailer for the 3ds game Pokemon Sun and Moon.
Mimikyu song lyrics.
*I do not own the content of this video. Original video was taken from ポケモン公式YouTubeチャンネル. I have translated the song lyrics and then subbed it.