If Marshadow's typing is Dark/Fire as everyone concludes based on the rumored leak, could it be so that its called the Chaos Pokémon, and as such, plays as the prime opposite to Zygarde the Order Pokémon?
That aside, I also like to believe that Mars, the said planet is included in its name, seeing as the Roman God of the same name embodies War, and goes well with the Dark type. In addition, Planet Mars is also associated with the Babylonian God, Nergal, who in turn is known as the god of fire, destruction and war, which could further explain the Dark/Fire typing that Marshadow may have.