From the Super Mario Wiki
A Dark Mechawful.5 (known as Dark Mechawful 1.5 in the British English version) is an enemy found in Mario & Luigi: Bowser's Inside Story. When Bowser destroys a Dark Mechawful and leaves the ejecting head alone for a few turns, the head becomes a Dark Mechawful.5, gaining a new metal body. The differences between the two are:
Other than those differences, defeating it is similar to defeating the original Dark Mechawful, including the need to inhale the head to prevent more Dark Mechawful.5s from spawning.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
Names in other languages