The first sheet I made was for a former-NCR cavalry group. It was revealed afterwards that horses were extinct. I had thought they weren't because there is an official comic showing horses, but apparently the guy who wrote the comic admitted later that the horses in it were a mistake, and that horses are actually extinct. 

When I said 'mechanized' I didn't mean main battle tanks. Maybe I should have been more clear. What I meant was basically a few trucks, and maybe armored personnel carriers and things like that. Their origin would have been similar to my first sheet, so they would have come from the NCR. Maybe a small group that broke off from their mechanized group after a certain event.

EDIT: I'll see if I can write something up.