Drops from Donggu (Funguar NM) in Ordelle's Cave. Pops around (H-7) on the same map of the strange apparatus. Lottery-pop from the Fly Agaric in the room.
You will need a Beastmaster in your party to unlock its latent.
As stated already this drops from Donggu, a Funguar NM in Ordelle's Caves, he is a lottery pop from a specific Fly Agaric at (H-7), it is the one that appears as the last in the list of 4 in widescan.