Normal: "A fiend that grows in damp places and releases a spore cloud when struck. It's hard to believe a runt like this can use Ultima..." Oversoul: "A fiend that grows in damp places and attacks with magic and spore clouds. When its HP gets low, it enters a sort of "Magic Berserk." It's not pretty." — Scan
The Mushroom Cloud is an enemy from Final Fantasy X-2. It is the most dangerous of the Fungus group, as it can inflict a lot of statuses with its Pernicious Powder, which it uses as a counterattack to whenever its damaged. In Oversoul, it becomes worse as it has access to Ultima.
This is probably why it's called Mushroom Cloud. It is HIGHLY recommended to take it out in one hit or else escape if that is not an option, unless the party has Ribbons equipped. Fire-elemental attacks are the way to go to bring down this fiend.