I haven't heard of the usual ingredients but for "dog's bread" has been made from bran.
it varies, but those mentioned since forever were  usually made from bran.
How did the idea of dog biscuit come about? Do you mean "dog's bread"? If so, then I'm not sure of how they came to be but know they've been around since the Romans. Wow so it has been around for hundreds of years! Yeah, but did you hear how manufacturers of it claim it cleans a dog's teeth and promotes oral health? No but I suppose anything to make a buck and I don't know but maybe it's true! Are dog biscuits shaped like the biscuits people eat? Well, theirs can come in a flat bone shape. What is a dog biscuit usually made up of?
''Dog's bread'', made from bran, has been mentioned since at least Roman times.