As Kash says the best way to get an idea of what they are is to unload that assets.bin folder, go to the "characters" folder and open one of the images to see the "Sprite Sheet".
After that you can also open one of the .xmls to see exactly how the options are set up in the XML code. XML for HW is very easy, but if you've never coded anything before I can understand how confusing it will be. There is currently a tutorial going on by Linaru that I believe is addressing some of the basics for the XML, but I'd be happy to help you out with any of your questions.
The first step will be to draw a sprite sheet for your new mob (and yes, mob is short for mobile which is short for mobile-ai-object I believe).
Take a look at the Tick sprite sheet, and maybe draw your mob using that as a reference. You will need:
8 directions of movement, and 8 directions of attacking. Idle and death animations are optional, but you can do those as well. Once you have that sprite sheet you can open the tick.xml and just replace the info for the animations there with the info for yours (replace tick.png with newmob.png, and change the coordinates and sizes to your sprite)
If you need help with that, let me know!
Once RW Act 1 is released, I'll be making some videos on some basic spriting and animating and such, with pointers and tips to the best of my ability. I'm happy to help however I can!