These friendly creatures show up most commonly around the tent in the Deep Jungle, the Treasure Room at the Magic Cave in Agrabah, and the large room just before reaching the Final Rest at the End of the World. They aren't hostile, but you have to hit them three times in a row with the same type of magic to get them to drop the Mystery Goo. The gestures they make will tell you what type of spell they want you to use on them:
Shivering - Fire
"Bring it" gesture - Ice
Sun over head - Thunder
Trip and fall - Cure
Spinning around - Aero
Frozen in place - Stop
Floating in air - Gravity