How did the idea of dog biscuit come about?
"Dog bread" as it was called was already mentioned since the Roman, so they have been around forever
Wow so it has been around for hundreds of years!
At least nowadays, dog biscuits tend to be hard and dry, manufacturers claim that is to keep dog teeth clean.
how can they get?
Chewing dry hatd thing can clean the teeth somehow.  Dog biscuit usually are sold in a flat bone shape.
What is a dog biscuit usually made up of?
it varies, but those mentioned since forever were  usually made from bran.