Excessive Spooking.
It is normal for horses to be wary or frightened of unfamiliar objects.
They are programmed by nature to be that way for survival purposes.
But when they are functioning normally it is easy to accustom them to new experiences by using a good approach.
Overview
In the picture above Sherilyn is helping Wizard to overcome his fear of ropes by carefully controlling the situation. She could pull the rope off of him if he became any more alarmed.
When horses are ‘grass-affected’ you can spend many hours on these kinds of things and make very little progress for the time spent.
It is not normal for them to spook at things that they are familiar with that they did not spook at last week!
What is Normal?
When a normal horse spooks it is ridable by the average rider.
When a grass-affected horse spooks they can literally leap out from underneath you, the next thing you know you have hit the ground so hard you can’t walk for a week! (and that's if you are lucky!)
It is easier the work with a Kaimanawa or a Mustang straight off the range than a spooky grass-affected horse.
The wild horse is relatively easily accustomed to unfamiliar stimuli in a very short time. The horse that is not functioning normally due to mineral imbalances and toxin overload will be far more expensive in terms of time, effort and risk.