You can obtain the Prison Bottle in any Poké Mart from the clerk in front of the counter once you have Hoopa in your party.
The Prison Bottle allows Hoopa to change from its Confined form into its Unbound form.
Getting the Prison Bottle (Step-by-Step)
You’ll first need to add Hoopa to your party, and this can be done using cheats to make it simpler. After that, just enter any Poké Mart.
This method works the exact same way in all Poké Marts around Hoenn.
Once you’re inside the Poké Mart, walk to the left side of the store.
Talk to the clerk in front of the counter here.
The clerk will then tell you a short story.
Once the story ends, the clerk will give you the Prison Bottle.
Using the Prison Bottle
With the Prison Bottle, you will be able to switch Hoopa-Confined into Hoopa-Unbound.
Step 1: Select the Prison Bottle from your bag’s Key Items’ section.
Step 2: Select Hoopa and use the Prison Bottle on the Pokémon.
Step 3: Hoopa will then change into its Unbound form.
Note: Hoopa can only remain in its Unbound form for three days. The Pokémon will then revert back to the Confined form.
In the Generation VI games, Hoopa will also revert to the Confined form when deposited in a PC Box.
You can change Hoopa back to the Unbound form by using the Prison Bottle again.
In the Confined form, Hoopa is a dual-type Psychic/Ghost Pokémon.
Hoopa-Confined has high Special Attack and Special Defense stats. The Pokémon works better as a bulky special sweeper.
On the other hand, Hoopa-Confined has very little physical bulk, and is also a slow Pokémon.
In the Unbound form, Hoopa is a dual-type Psychic/Dark Pokémon.
Compared to the Confined form, Hoopa-Unbound has a much higher Attack stat. This form also has a slightly higher Special Attack stat.
Hoopa-Unbound can be used as a physical, special or mixed sweeper.
On the other hand, the Unbound form also has a low Defense stat. And despite gaining 10 stats points in Speed, Hoopa-Unbound is still relatively slow.