Mystery Goo is a material found in Kingdom Hearts that is used in Item Synthesis. It is used in the creation of several high level items, but perhaps most importantly is used in making Sora's Ultima Weapon.
Mystery Goos have one of the more unique, and perhaps one of the most frustrating, ways of dropping in the game. They will only drop from the three mushroom-type Heartless, and then only if certain conditions are met.
Drops
White Mushroom
White Mushrooms are the most common mushrooms found in the game, but perhaps have the most annoying drop mechanism in the game. You always have a 10% chance of a Mystery Goo dropping upon appeasing a White Mushroom with three Magic spells. In order to increase the chances, you must cast the same spell three times in a row to satsify the Mushroom's demands; the chance of Mystery Goo dropping then increases to 20%.
Rare Truffles
Rare Truffles are appeased by being hit in air a certain number of times. The minimum times you must hit a Rare Truffle is ten times in a row; this gives you a 20% chance of recieving a Goo. Hitting one fifty times increases the odds to 40%, while one hundred times guarantees 100% a Mystery Goo will drop.
Black Fungus
Black Fungii have the most straightforward way of dropping Mystery Goos; you must defeat them with a Critical Hit. Due to their unique defense and way of fighting, this can be a tough thing to pull off. If Munny drops from a Black Fungus on the last hit, you are guarenteed a Mystery Goo as an item drop.