The FEAR System
The FEAR system is our body's way of protecting us from physical threats. Our horses wouldn't be alive today without an active and well functioning FEAR system. By nature, horses are neophobic (afraid of new). When presented with something new their first assumption is fear. This is how they have survived for centuries in nature and domestication. This has always been an important feature in their evolution, the ability to sense and respond appropriately to fear.
When presented with a scary situation we have a number of ways to deal with it. We often think of fight/flight, which is commonly understood responses to fear. Horses often resort to flight, and humans have used that as a tool to train and control these animals through their well developed FEAR system. Some are more prone to fight, we see this in "cowy" horses, who chase the scary thing. But we can't forget the others....
Freeze we see alot with horses it's usually the first moment people notice their horse is scared. The horse locks up and focuses in on something, frozen in place. They are assessing and deciding how to handle the situation. Punishing this is common but can lead to the horse jumping faster to fight/flight rather than thinking first.
Fawn is usually when a horse is afraid of another living being, even a friend, is threatening their physical safety. They try to soothe and comfort their peer, look submissive and gentle and offer appeasement gestures to reduce the risk of harm to themselves. This is like people pleasers, afraid of confrontation, agreeing to avoid a fight...
Fidget is how it sounds, we see this alot in horses who's ability to fight or flee is taken from them, it comes out in wriggling, pacing, weaving, squirming, dancing, just trying to release that pent up fear and stress.
Flop is another with little understanding, this is when we are overwhelmed by fear we lose our ability to control our body, we collapse, become disoriented, just check out of our body as a whole. This is Tonic Immobility. Which is frequently, cruelly, exploited in training.
We, and horses, and all species, are born with a natural set of known fears. We are genetically programmed to protect us from the dangers our species might often face. Some are even universal, sudden loud noises, pain, extreme heat or cold, the dark... these are called "unconditioned", we just know these things are scary, we didn't have to learn it.
We also have "unconditioned responses", these can be reflexive, uncontrollable, instinctive reactions, like flinching, wincing, gasping... we also have "instinctive behavioral action patterns" which are behaviors our body knows how to do in response to specific scenarios. In relation to fear this would be the 7 Fs. Some we see often in horses is kicking out when something is in their blind spot (while they are in a fearful state), bucking when something is on their back, stomping scary things on the ground, etc...
The FEAR system, as all systems, doesn't just turn on and off, it has levels and degrees and blends with other systems. The fear inside our body grows as the scary stimulus maintains, it subsides as the scary thing goes away. It adds to itself if an additional fear is added, making it harder to control ourselves through.
Then we have Conditioned or Learned fear, these are things the horse has learned through their life experiences to be afraid of. Its taught through classical conditioning, pairing the unknown with a known fear inducing stimulus. A horse may not be born knowing to fear an electric fence, but one zap and they quickly learn! Often this conditioned fear response is used to train and modify behavior without needing to use extreme physical force or control, since humans can't outmuscle a horse they use conditioned fear for control. So, a gentle tap of a stick paired a few times with a hard hit of the stick, the horse quickly learns that gentle tap=hard hit. Even a non-tactile gesture can have a conditioned fear response. They learn to react to the conditioned stimulus as if it were the real thing.
FEAR is generally an unpleasant feeling which we (and horses) work hard to avoid. So its through applied negative reinforcement people use this for behavior modification.