The official Japanese Movie 18 website has updated, revealing the official title of the feature film as The Archdjinni of Rings: Hoopa (光輪の超魔神 フーパ ) and the accompanying movie short's title as Pikachu and the Pokémon Musicians (ピカチュウとポケモンおんがくたい). Translations follow:

Story

Aiming to become a Pokémon Master, Satoshi continues his journey together with his friends.
Just recently, they were visiting a Pokémon Center in a desert.

As they were taking a break and eating some donuts, a mysterious ring suddenly appeared behind them.
Then, a hand extended from inside the ring, attempting to take a donut!
"Wait!" Satoshi and Pikachu both grabbed hold of the hand,
which then pulled them both into the ring.
The ring then disappeared,
leaving a dumbfounded Citron, Serena and Eureka behind...

"Appear!" Satoshi and Pikachu fly out of the ring!
The scenery has suddenly changed, confusing Satoshi.
"Hihihi! Are you surprised?" "Who are you?" "I'm Hoopa!"
Huge towers can be seen in the distance.
Incredibly, as Satoshi and Pikachu passed through the ring,
they were instantly teleported to Désser City, the destination of their current journey.

This is the first time they encounter the mysterious Pokémon Hoopa.
What is this Pokémons true intentions?
What kind of powers do its rings conceal?
This is the beginning of Satoshi, Pikachu
and Hoopa's adventure!

Character Profiles

A new mythical Pokémon debuts in the movie!
Its name is Hoopa!


By speaking the word "Appear!", it is able to summon all kinds of things, Pokémon and people included, with its rings!
It loves pranks and donuts!



Hoopa
Species: Mischief Pokémon
Type: Psychic/Ghost
Height: 0.5 m
Weight: 9.0 kg
Ability: Magician

This is the first time they encounter the mysterious Pokémon Hoopa.
What is this Pokémons true intentions? What kind of powers do its rings conceal?
This is the beginning of Satoshi, Pikachu and Hoopa's adventure!

Picture	Biography
	Primal Groudon: Species: Continent Pokémon Type: Ground/Fire Height: 5.0 m Weight: 999.7 kg Ability: Desolate Land
	Primal Kyogre: Species: Sea Basin Pokémon Type: Water Height: 9.8 m Weight: 430.0 kg Ability: Primordial Sea
	Black Mega Rayquaza: Species: Sky High Pokémon Type: Dragon/Flying Height: 10.8 m Weight: 392.0 kg Ability: Delta Stream

Bookmark Campaign

Pokémon movie information will suddenly "Appear!"
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We're constantly revealing new information on the Pokémon movies!
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Posters

The poster for the short got updated with its newly revealed title:



Sources: http://www.pokemon-movie.jp/, http://www.pokemon-movie.jp/story/, http://www.pokemon-movie.jp/chara/ and http://www.pokemon-movie.jp/topics/bookmarks_campaign.php