The difference is the cooking time. Quick oats cook in 5 minutes and the regular oats cooks in 15 minutes. That is why they are called Quick Oats. The reason that the cooking time is different is that quick oats are regular oats that have been cut a few times to make them smaller pieces.
The typical ration of oats for a horse is usually between 2 and 6 pounds of oats a day. If you are feeding more then 3 pounds of oats it is recommended to split the oats into two or more meals throughout the day.