When a horse is bored, he/she will find ways to amuse him/herself
so in the stable they may develop habits such as cradle rocking
(swinging the head from side to side) or crib biting (continuing to
bite on a surface - usually wood). The horse also may because
aggressive and bite/kick through boredom. For stabled horses, it's
best to have a constant supply of food, a toy and frequent riding.
If there is a pasture available, make sure the horse gets time to
spend outside, as it is natural for a horse to be outside and they
will feel a lot better.
In the pasture, crib biting is also fairly common but horses who
live in a herd outside are less likely to become bored than a
stabled horse, unless the horse has been ignored for a long period
of time or is in a field too small..
When riding, horses show boredom through laziness, bit biting,
refusing to do what's asked and generally being nonreactive to
tasks asked. It's best to include many different things while
riding to add fun to the ride and make it more interesting for
them, similar to humans.