so...you wanna know how to create a new monster by your self?
well,
first you should get your hands on a 3D model program. such as; Blender(this one is free), Maya, 3DS max etc.
i recommend you to look some basic tuts on youtube
you should know how to model and texture (Unwrapping)
after you've completed these things, the hardest part is coming and that is Animation.
this category is still hard for me. so i won't give any false information
you could see video's on youtube for this also.
bad thing about animation is that the game only supports Maya for animation.
Now you can put everyting into the game. For this you should use Model Editor. I recommend you to look at the Grunt's settings. look at the variables settings for the monster settings. And you can add animation in the animation tab, these should be the same as the grunt. also make sure you've made a .mat file whith the material editor. after you can save it as a .ent file and put it whith the Lever Editor in-game
this is just a non-detailed explaination. first try to make a simple model :Angel:
Good Luck!