Insane Difficulty: The ultimate game content / spoiler / Q & A thread! (V8.0) - Insane Difficulty

luka megurine provides found illusion

Rose Way: Final Screen - Amazy Dayzee they drop In-Battle Golden Flowers (Rise FP by 3) And Outside of battle, Golden Petal (temporary rises FP by 3)

Forest Maze: In the Maze you can find Brachiosaur. They Drop outside of battle "Twin Stars" (can be turned into Three Stars by Rikku. Mixed with a Gambler's Spirit)
In the Forest Maze Secret Area, you can find Maenad (AFTER you beat the Misterious Girl. There are 2 type of them. Altho they look the same). They Drop Crystal Ring outside of battle.

In the Ice Cave, in the same room you fight Shiva, you can find the Blue Dragons. They Drop The Ice Mail outside of battle.

In Molevile, in the room where Punchinello was, you can find Tonberrys. They drop in-battle Tonberry Dolls. (Can be turned into Ribbons by the Mole that gave you the shiny stone in Vanilla). And Outside of battle, they drop Gambler Spirits (For mixing items with Rikku)

In Star Hill, you can encounter Don Tonberry in the same room Godess was. They drop L. Shroom Shake 3 in Battle. And Healing Springs outside battle (Cast Regan on 1 pary member. Also can be mixed by Rikku into one of the best items of the game: The Hyper M. Guard)

In Seaside Town Secret Area you can find Long Gui. They have high Hp and can drop Gold Bars In-Battle, and "Trapezohedron" Outside of the battle. That item can be used to upgrade armor with Magikoopa in Bowser's Keep.

In the second room of the Sea, you can find Gold Dragons. They Drop Ultra Shrooms in-battle and the Diamond Mail outside of the Battle.

In Via Infinito you can find in the first room (after you beat the M. Tonberry) well... more M. Tonberry. They Drop Kerocola3 in-battle and Dark Matter outside of battle. (Inflicts Fear for 3 turns. Can also be mixed with Rikku)
In the second Room of Via infinito, you can find more Elder Drakes. They drop Hyper Wrist outside of battle. (Accessory that Greatly increases Attack)
In the Third Room of Via Infinito, you will find Shadow Dragons. They drop Ultra Shrooms in-battle and the Demon Mail outside of battle.
In the fourth room, you fix-batlle Chac. He drops Adamantite (Accessory that Greatly increases Defense)
In the fifth Room, you can find more Chac, but they drop Magistral Rings (Accessory that Greatly increases Magic)
And, in the Sixth and last room of Via Infinito, you will find White Dragons. They also might drop Ultra Shrooms in-battle and the Crystal Mail outside of battle.

In the new area in the Land's End Desert (just before the cliff is the new opening) you can battle Cactuars. They drop Kerocola3 in-battle and Speed Bracer outside of battle. (Accesory that grants 100 speed)

In Barrel Volcano, in the room where you fight Kaiser Dragon. You can find Red Dragons. They drop Ultra Shrooms in battle and Flame Mail outside of battle.