horses are omnivores??

Are you saying that horses eat meat ? Here is a good explanation of the difference in herbivores, omnivores, and carnivores:

Humans are omnivores and herbivores are derived from omnivores.

There's two things you have to look at: the mouth and the gut. And to do this you have to go back to where the placental mammal group (Eutheria) diversified into seperate lineages or species. The first to directly break off from this lineage were insectivores (anteaters and armadillos). Carnivores are another seperate lineage that led to dogs, cats, bears, raccoons, etc. Then came omnivores that still had canine teeth to strike at, attack, and kill their prey.

Most herbivores today have no canines because they have no need to subdue their food , but instead have an extensive array of thickly enamel-covered molars and premolars and jaws that move sideways to grind up their food, rather than to just crush the food into a small pulp like omnivores do (mastication). In cows, horses and deer, the dentine (teeth) structure is "lophed," meaning that they have ridges in them for grinding. Human teeth are not as lophed as cows, horses, and deer. We also do not have the extensive masseter muscles as they do that allow the back rows of the teeth and the back of the jaw to move sideways with such extensive grinding force.

The second major difference between omnivores and herbivorous is the gut (stomach and intestines). We no longer have a cecum - a seperate stomach chamber - that allows fermentation to break down and digest cellulose (plant fiber). The remnant of a large cecum (or plural ceca) in humans is now our small thin appendix. Humans are unable to digest cellulose like cows, deer, horse, elk, sheep, and rabbits. Even a monkey has a much larger and longer cecum than what is now leftover as our appendix.

In general, all herbivores have a much larger stomach, larger cecum, and a much larger and longer intestine than omnivores. Ruminants (cattle, sheep, goats, deer, and giraffes) have an extremely large four chambered stomach that they use to digest the cellulose with special microorganisms and enzymes (cellulase) that humans and carnivores do not have.

Carnivores have an extremely short intestine because the food that they eat - meat - has already been digested by the original herbivore that they ate. Carnivores include amphibians, snakes, crocodilia, hawks, and carnivora - dogs, cats, bears, raccoons). Even though the former do not have canines (because their prey is so small that they not need canines to seize and hold their prey in the same way as carnivora need to do), they all have shorter and unspecialized intestines, and lack the enzymes needed to digest cellulose. However, it is thought that a small amount of cellulose digesting enzymes are still produced by the human appendix. Thus we have gone through a transition. But humans are not carnivores because we do not have canines; and our intestines are in general, longer than carnivores, yet much shorter than herbivores.
Carnivores only eat meat: herbivores only eat plants. We are neither.

We are omnivores because we eat both, and because of the structure of our teeth (incisors, premolars, molars, and lack of canines), the way that we masticate (chew food into small clumps called a bolus, before swallowing), the way our salivary glands lubicate the throat to allow us to swallow, our lack of any significant cecum to produce the enzyme cellulase to digest cellulose, and the mid-range size of our stomach and mid-range length of our intestines that allows us to digest some plant nutrients, but not to adequately digest raw meat, as carnivora can.