On the question of "what the hell is a Muto" I have come to the conclusion that placoderm fishes would be the best choice for an ancestor. Mutos are mostly vertebrates, but the number of limbs and the jaws make them superficially resemble arthropods to some degree. However, using fishes as a base, both the jaws as well as the limb number can be explained. In this case, the outer parts of the lower jaws were assumed to be mandibles, while the middle part is a separate element developed from a gill arch, and the three front limb pairs are the result of the pectoral fin splitting into several parts and developing into separate limbs, similar to the "legs" of a sea robin. Placoderm fishes and especially arthrodira were chosen as the base due to the armoured front body and especially the pattern of ventral shields matching up.
Development: http://www.furaffinity.net/view/15578945/