I have always felt that the horse does not get concerned overly if it can hear or see something approach. To be startled suddenly is another matter. It is often the case that a rider [who very often who does not hear the aircraft approach] gets startled and transmits that nervousness or concern to the mount.
You are right - to a degree! It depends on the individual horse - and the individual 'something' - and each horse will view that 'something' differently depending on his life experience, training and natural temperament.
The horse evolved as a prey species - flight is his first defence. And ANYTHING he is not familiar with is a potential predator. Now many horses don't react to an aircraft (or a fast car) because while they are obviously 'predators' they are gone so quickly that they are obviously chasing some other poor bu**er! If the 'something' lands - then a loose horse will put a 'safe' distance between himself and it - then stop and watch. If 'it' doesn't move, he will return to investigate (and never leave a light plane or chopper unattended in a field with horses - they'll strip it bare!
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But a plane or chopper that hovers around above horses is NOT chasing someone else - it's waiting to pounce - and making a lot of noise in the process! If the rider is capable of stopping the horse from running, it is likely the horse will go into 'fight' mode; rearing and bucking to make himself appear bigger and more frightening!
And yes - in many cases it is the rider's fear of what the horse MIGHT do that causes a horse to panic. But if it's your 10 year old on a pony that's bolting towards a busy road, who are YOU going to blame?
They should cross breed all horses with Dartmoor ponys. You have to practically land on the things before they will look up, glare at you and wander about 3 feet away before going back to eating whatever it is they find so tasty on those blasted heaths.
I suspect that's familiarity as well as temperament - Dartmoor is used by the military for low flying training (when they're not above my farm!! What ARE those HUGE dark, quiet planes that travel very quietly and slowly??)