From the Super Mario Wiki
The Malodorous Fibbian is an item in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. Its name is a play on the word "amphibian." According to Dieter, it is the most toxic organism in the Mushroom Kingdom, with poison 100 times as potent as that of a Poison Mushroom. The Best Fitness Friends combine the frog's breath with Skeletone Formula:D to turn it into Malatone Formula:X and then feed it to Morton. This causes him to become hostile to Bowser Jr. and his minions, thus throwing them right into the ocean of Plack Beach.