Hoopa

Hoopa Confined is a light-purple Pokémon with a vaguely humanoid appearance. It has green eyes with yellow sclerae, a yellow ring marking on its forehead, a small, pink dot under each eye, and dark pink hair tied into a short ponytail. On either side of its head is a dark gray horn that curves slightly upward. Its body has a dark pink, V-shaped marking on the chest and a small, golden spike on each shoulder. Its arms are not attached to its body and instead float next to it; each arm has a small golden spike similar to those on its shoulders. It has mitten-like hands and dark pink rings around each wrist. In place of actual legs, it has a wispy tail. Hoopa has three gold rings: a large one around its waist and one hanging freely on each of its horns.

In the past, Hoopa's true power is said to have been sealed away in the Prison Bottle for the destruction it used to cause, reducing Hoopa to its smaller, Confined form; opening the bottle grants Hoopa its true form once again, but for only three days. In Hoopa and the Clash of Ages (M18), the one who sealed Hoopa's power in the Prison Bottle was a traveler named . Hoopa's rings can open Hyperspace Holes, which warp space and allow it to transport objects, humans, and Pokémon, including even Legendary Pokémon, by passing them through them. As seen in Hoopa and the Clash of Ages, summoning too many Legendaries has the dangerous side effect of creating a space-time distortion that'll destroy the area where they were summoned. As seen in various media, these holes allow it to access other worlds such as the Distortion World/Reverse World, 's world, 's world, Hyperspace Lumiose, and even parallel timelines. With this power, it can cause mischief and take anything it wants for itself. Hoopa is connected to many strange events because of these mysterious powers. It is also said to have the power to bend dimensions. It is heavily implied to be the one causing many of the mirage spots in the Hoenn region, specifically the ones featuring ring-shaped portals.

Hoopa is very fond of foods that have holes in them, especially donuts. By consuming certain donuts, such as the various types made by Ansha, it is granted a great surge of power, enabling it to influence both humans and Pokémon in dimensions such as Hyperspace Lumiose. One of these influences is allowing Pokémon to be up to Lv. 200 in hyperspace. Despite not being able to Mega Evolve, by eating a donut infused with the power of Mega Evolution, Hoopa can have some of the power of its original form awakened. This allows it to do feats seemingly beyond its Confined form's power and on par with Mega Evolved Pokémon.

Hoopa's lair is rumored to be located inside an hidden desert oasis. Its lair contains many objects and treasures Hoopa obtained over the years, claiming as its own. Hoopa's lair is believed to contain mountains of gold and silver treasures it stole from ancient kings and wealthy land barons. To the present day, no one has ever found Hoopa's lair due to how well hidden it is. Hoopa stores all of its gathered loot in a secret spot only it knows, which appears to be its aforementioned lair. Hoopa can be very vengeful to those who attempt to manipulate it for its unique power. According to one tale, it punished an organization by transporting their individual objects before moving their headquarter to the middle of the desert. This incident terrified the organization to the point of disbanding.

Forms
Hoopa has two forms: Hoopa Confined and Hoopa Unbound.

Hoopa Unbound is a large, bipedal Pokémon with six arms. It is mostly light blue with purple hair. The ponytail on its head is larger and bushier and held in place by two golden rings. Its hair now covers its chin, where it forms a pointed beard, and the back of its neck. The horns are less curved and have spiky purple hair around the bases. There are three gold rings on each horn: two around the base and one around the middle. It has yellow eyes white sclerae, a pointed, dark gray nose, a small hole inside of the ring-marking on its forehead, and prominent fangs in the back of its lower jaw. Similar to Hoopa Confined, its arms are disconnected and float next to its body. Instead of a small, gold spike the arms now have large, dark gray spikes and appear from holes along the sides of its body. Each arm has three fingers with dark gray claws, a gold ring around the wrist, and a gold ring around the base. Several tufts of purple hair surround its torso and a gold ring encircles a hole in the center of its chest. The hole in its chest is said to lead to another dimension. Another large gold ring encircles its waist and there are small tufts of hair poking out from underneath it. There is a purple stripe that widens at the knee down each leg and two gold rings around each ankle. The tips of its feet are gold and curl upward, resembling shoes. Additionally, this form has a tail with a purple stripe along the top and an arrow-shaped tip. Hoopa Unbound is tied with Mega as the tallest Dark type.

Hoopa Unbound is said to be Hoopa's true form and is said to have a humongous amount of power. Hoopa is at full power in its Unbound form, it can increase the size of its six rings to grow multiple miles in length. These massive rings are capable of transporting entire islands through its hoops. Hoopa Unbound is capable of bending dimensions on an even more powerful scale. As depicted in Hoopa and the Clash of Ages, it can use its own rage to drive Legendary Pokémon berserk. One legend stated that it greedily teleported an entire castle from Kalos 3,000 years ago to obtain a treasure hidden within it. Hoopa can remain in its Unbound form for only three days, until its power is sealed back into the Prison Bottle, reverting itself to its Confined Form.

(Source: Bulbapedia)