From the Super Mario Wiki
Stonks are purple cloud-like enemies found only in the Airway in Mario & Luigi: Bowser's Inside Story. They have been influenced by the Dark Star. When Mario and Luigi fight these enemies, they may split themselves into four smaller Stonks, and circle the bros.. If they do not hammer quick enough, the clouds will strike, and possibly poison them. However, the bros. would have to hit the right one – if the bros. hit a different one other than the leader, the cloud will dissipate. They will also attack by forming new clouds and blasting them at Mario and Luigi. This attack needs to be avoided by jumping. This attack also has a high possibility of poisoning the bros. They are usually paired with Air Cannons and Airnapses. Stonks are weak to fire, so it is recommended to use Fire Flowers against them.
Names in other languages
|From 「匂い」 nioi, meaning scent.
||From gas and peste (stench).
||Pun on nauséabond (nauseous) and cumulonimbus (cumulonimbus cloud).