Cereals grains have served as an energy dense horse feed for ages. However from evolutionary standpoint, horses are not equipped to digest large amounts of starch. Due to this fact, health issues are very common amongst horses that are being fed large amounts of cereal grains. Even though grains are not unhealthy to horses by definition, there is a lot of proof that horses do better on medium to low starch rations. And in some situations it may be even better to lay off the cereals alltogether.
An article written by Marike Jacobs (in Dutch) for the Dutch horse nutrition platform VoerVergelijk.nl.