If horses ears are slanted backwarrds what does it indicates he is?

It depends on the situation the horse is in. If the horses ears are swiviling back and forth and moving around alot they are more than likely just listening to sounds, other horses or you.

If their ears are pinned back against their head and their teeth are bared (like a dog snarling) then they are upset or angry. Their tail may also swish around if they are angry. Horses pin their ears back as a warning to other horses or things they feel are a threat.