Why can you understand Scrappy Doo, but you can't understand Scooby Doo?
I don't know.  I guess because Scrappy Doo was a mutation or something like that.

Why did Shaggy like to eat dog biscuits?
I guess they taste good, most likely because Shaggy was always hungry.

Did they ever say the last names of the characters?
I can't remember.

Who was your favorite character? Why?
Velma.  She was the smartest member of the group.  She was the one who really solved all the crimes.  The others were there as window dressing.