No you absolutely cannot feed horse meat to another horse. Horses are strict herbivores and cannot process meats. There is also the possibility that the meat could make the horse sick.
Many things can be used to feed both horses and cattle, grass is first and foremost what they are designed to eat. Other food stuffs would be hay, corn, oats, barley, haylage, and processed feeds made by a manufacturer.
The same thing as other horses. Hay, grain, horse feed, etc.
There is no one amount that all horses should eat. A horse only needs sweet feed if it needs more energy to complete it's daily job. A horse should eat between 1.5% and 3% of it's own bodyweight daily and the vast majority of that should be forage such as hay or grass.
Cattle and horses eat crabgrass.
It depends on the average size of the cattle. The amount of feed to feed cattle depend on their average weight and sex of cattle. Mature cows tend to eat more than weaned calves; young bulls eat more than steers, and steers tend to eat more than heifers. Older cattle eat more than younger cattle, and, thinner cattle eat more than fatter cattle. So with that, I cannot give you an accurate number.
Yes, horses can eat rolled oats as part of their diet. However, it is recommended to feed them in moderation as too much can lead to digestive issues such as colic. It is also important to ensure the oats are free from any contaminants or additives.
From the feed they eat and how it gets digested in their bodies.
Don't feed horses rotten apples
To both questions, NO. Horses and cattle are two entirely different species of animals, and should be treated as such, even though they are found in the same environments i.e. ranching and farming and eat nearly the same thing, i.e. grass.
Feed them alfalfa
Horses get their food in the same way, no matter their color or breed. Wild horses graze and domestic horses graze and wait at the feed trough. Color is irrelevant to horses and their stomachs.
Horses in Minecraft eat hay. This can be made using wheat. You can also feed them apples.