The curious thing about the myths and legends surrounding Hoopa is that, while this pokemon is said to be as rare as Mew or Celebi, it was never valued for its own sake. Some of the greatest trainers in human history have tried to catch Hoopa, but each of them only sought it out as a means to an end. For Hoopa's three magic rings grant it the power to summon three more pokemon – god or mortal, from anywhere in the world. This pokemon was not honored in its own right by those who captured it, but as a granter of wishes, a djinn of the rings.
Hoopa's trainers never sought to use it in battle; had they done so, they would have discovered a pokemon with power exceeding virtually anything they used it to summon, one capable of manipulating hyperspace to strike anywhere in the world. Instead, they have only ever learned these facts the hard way. Legends tell of Hoopa's captors (or sometimes those around them) being tricked into opening the bottle which confines its power, and the rebellious Hoopa does battle with the very pokemon it summoned – a battle usually ending in its victory, and the trainer's death.
Should you ever seek out or come across the mischievous Hoopa, I beseech you to give it a chance to demonstrate its own power, and treat it as a valued member of your team – not merely as a way to summon another one. Trainers throughout history met unfortunate ends when they strove to control a power they neither respected nor understood – yet Hoopa's rage is not infinite, but a curse stemming from bitterness, neglect, and a desire to show others what it can do.
In caring hands, it can triumph over the world.