Mortrons are an endangered, possibly extinct, species that hail from a planet whose sun when nova. They were first discovered by Yeerks, and when it was discovered that their brains were too small to be suitable hosts, the Yeerk who would eventually become Visser Three took two as pets.
Mortrons are fascinating because they are comprised of two parts, though usually both are connected. In their full form they are three feet high and four and a half feet wide. Most of their body is a dark, dirty yellow with irregular black spots, but their shoulders and head are a dark red. Their heads are remarkably small and pointy, almost needle-sharp. Their mouths are long and narrow, and dozens of irregular teeth jut from them at varying angles. They do not have feet in the traditional sense, however; Mortrons are equipped with four wheels, one where each of their legs might be. Their wheels are sloppy and imperfect, yet functional and effective.
When a Mortron has chosen a target, they charge, making a "HUF-HUF-HUF!" sound as they do so. When they are close enough, the upper, dark red portion separates from the lower body, unfolding wings it presumably keeps beneath itself, and flies at the target, attacking with its sharp teeth. The lower half does not seem to participate in fights, instead choosing to veer away from the target. It is unknown if it is given this propulsion before the top half separates, or has a mind of its own.
If a Mortron is segmented into two pieces, one of the halves will simply regenerate into the bird-like entity it was, while the other will begin the longer process of regenerating the entire base-top combination. It is unknown how the halves decide which will be which. It is also unknown what would happen if it were segmented into more than three parts. Visser Three claims that this ability is granted to them due to an enzyme released by the Mortron when it enters a "killing frenzy." This regenerative property makes them particularly effective against Andalites, however, it appears as though regular methods of killing that do not involve segmentation (e.g. bludgeoning, strangulation, etc.) are still effective, and will render a Mortron dead.