I'm a professional horse racing handicapper.
and I'm a degenerate gambler. what a coincidence. I do love that equestrian performance sport.
When was horse racing invented?
It's the most ancient of all sports . It has been around since time started and the sun started to rise in the sky. 
How much money is generated from gambling on horse racing?
More then you can imagine. In 2008 alone it generated over 115 billion. Yes billion. You must be rolling in the money with your handicapping skills. 
You know it. Are there different types of horse racing?
Yes, Horse races vary widely in format from country to country. 
Is betting on horses legal at all horse tracks?
I don't know. I've never been to all the tracks. I think people have been wagering on races since Ancient Greece, Babylon, Syria, and Egypt.