This guide was created with reference to the walkthrough site listed here.
https://appmedia.jp/stray_children/78424778

I’m translating Older chests information from Japanese using ChatGPT. I was able to confirm the older names in-game, but place names are harder to verify. I think the writing itself is a complete mess. I’d really appreciate it if you could point out any mistakes!

Appears randomly while enraged.

A treasure chest may appear beneath the main body during attacks in its enraged state.

A treasure chest may appear if you hit the ball with perfect timing during its enraged attack.

When Ribbigger attacks, a treasure chest containing gems may appear on the opposite shore.

A treasure chest appears after defeating the Mothership.

Appears after stomping on two slimes that spawn on the field during its attack.

If you reach the enrage attack phase without taking any damage, a treasure chest will appear during the enrage attack.

The treasure chest in the Lilithorn battle appears if you follow the correct sequence without taking any damage from the start of the fight.
Because you must avoid all damage up to the middle of its enraged action that occurs after the third choice, the chest is relatively difficult to obtain.

The treasure chest in the Hexabelle battle appears if you follow the correct sequence without taking any damage from the start of the fight.
Because you must avoid all damage up to the middle of its enraged action that occurs after the third Ruby choice, the chest is quite difficult to obtain.
The chest contains the key item “Estate Invite.”

The treasure chest in the Majora battle appears if you follow the correct sequence without taking any damage from the start of the fight.
Because you must avoid all damage up to the middle of its enraged action that occurs after the third Ruby choice, the chest is quite difficult to obtain.

The treasure chest in the Desert Fox battle appears during the final enraged action if you follow the correct sequence without taking any damage from the start of the fight.

To obtain the treasure chest behind the Guter's Door, you must first acquire a key during its normal attack phase.
Among the countless flying keys, there is one with a slightly different shape — touch that one to obtain the key.
After obtaining it, inspect the door that appears during its enraged attack to unlock it, revealing the treasure chest.
The chest contains a "Stuffed Doll", a key item required to help the later-appearing Older Honey Queen pass on.

The treasure chest in the Geargrind battle appears during the enraged action that occurs after the third choice, if you follow the correct sequence without taking any damage from the start of the fight.

The treasure chest in the Sokobanban battle appears randomly after you finish organizing the warehouse.
It’s probably triggered after you’ve organized all types of warehouses.

In “Trip to the Moon”, enter ↑↑↓↓↑↓↑↓←→← with the D-pad and clear it to make a treasure chest appear.

The treasure chest in the Gastrobaron battle appears when certain conditions are met during its enraged attack.
Collect the five fruits and the key that appear along the central path — once you pick up the key, or during the next attack, a treasure chest will appear on the field.
Your fruit collection progress does not reset even after the attack ends, so keep enduring its enraged attacks and collect the fruits one by one until the chest appears.

The treasure chest in the Blackman battle appears if you keep dodging the bats during its enraged attack without triggering the roulette.
The difficulty is extremely high.

The treasure chest in the Mimickun battle may appear at the start of its enraged attack.
Its appearance isn’t affected by whether you’ve taken damage beforehand — it’s purely determined by chance.

During its enraged phase, the number of hand signs increases up to 10 as you keep clearing the attacks. After successfully clearing all 10 in a row, a treasure chest will appear.