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.
Names in other languages