Most people think of mechs as fifty-foot tall shambling hunks of steel, blasting out 250mm death with impunity. Not many people realize how important speed is in warfare when they do mech designs, and it's just as easy to jump out of the way of incoming fire, and better even, than to take it head on. Your design practically oozes this - from appearances alone it's quick and light on it's feet, easily switching from recon to quickly engaging a target before bouncing away. I can see why 343 wanted to use it, awesome job, and excellent work on the picture itself. You're a very talented artist, Debris.
Do you have a name for this design? I'd actually like to use it in a story idea.
I forgot to iterate - though I have plans to create an original work in the future, I mostly write fanfiction, and I current fixation is on RWBY. This has no bearing on whether or not you let me borrow your work, but I felt I needed to fully explain my intentions, as I have no desire to steal or take credit for an excellent piece such as this.
Whatever you decide, this is an excellent piece, and good luck with your future works.
I congratulate you for letting 343 Studios use your concept design for the making of Halo 4's Mantis! Allot of people on Halo Waypoint pointed to you here on DA and others on 343 forums including 2 Halo 4 lead designers said they used & borrowed a good majority of parts from your concept design to make the Halo 4 Mantis.