So has anyone found all of the Behemoths without using the guide? I've only found one other than the brute that you get to fight once you find Galaxy News Radio.
If you go due north of Tenpenny Tower and keep looking to your left before you get to the river you'll see a big pit of white rocks with a railroad track running through it. If you head into the ravine you'll eventually start seeing traincars with landmines on either side of them. Turns out to be a pretty big raider camp complete with a few slaves to free (if you're into that sort of thing).
The real surprise is that there's a fucking behemoth that they've CAPTURED and it's being held in a big pen with an electrified door. If you shoot the generator right beside the door then it'll escape and you can readily expend all of your ammunition trying to kill it.
For every moment of triumph, for every instance of beauty, many souls must be trampled.
Hunter S. Thompson