What holds horses heads uncomfortably high?

This is likely the bearing rein, used in horse carriages. The bearing rein was used to force the horses head into an upright carriage to make it look more stylish. However it had bad physical side effects on the horses, including pinched nerves, muscle pain, back problems and breathing trouble. There are other devices that can be used while the horse is being ridden that work similarly to the bearing rein, these include the De Gogue among others.