I am not sure about all herps. But I know about horned frogs, as I have one. I know that in the wild, these frogs will basically shove anything into their mouth that will fit, and that they can catch. This includes insects, other frogs, possibly small snakes and other creatures even. It is comical in a sad way, because when they congregate for breeding purposes, you can often see frogs biting into each other and trying to eat their potential mates. And these guys have a JAW! I got bitten by one of mine years back, and it drew a lot of blood and hurt like heck, like small saw blades cutting into my finger.
I know that in captivity, most people just feed crickets to juvies. Thus, vitamin powder is used, I guess because the frog is not getting the wide variety of foods that they get in the wild (including other frogs!) I feed my adult mostly rats now with occasional nightcrawlers. Many people's young horned frogs die as a result of calcium deficiencies and the weakened, deformed bones that result.