I don't know that I can interpret fear in a horse. I can know what I interpret as fear;however, I am not sure that is the horses genuine emotion. I have seen horses react in what their owner calls "fear" to the vet and that behavior ends up being more aggression and the horse is dangerous. If the choice is between someone getting killed and putting the "fear of god" in that horse. I don't bounce nearly as well as 1000 pounds of horse does. So, having been in certain situations where it was use fear as a respect tool to keep that horse from not seriously injuring someone its appropriate. As others have pointed out, horses are really surprisingly tough. Look at the way yearling stud colts play with one another (rearing, striking, biting, bucking) all things that if they did to me could result in a case of deadness. I don't want to be my horses equal. My horse treats its equals like horses, I want possibly a mother foal relationship. I am the trusted, guardian/food source/ authority in the horses life. My word is the word.
I also don't know that I would hear fear as respect. I think that we can't interpret fear because it is really close the the "he was abused" story line we hear a lot. Which is used to excuse a lot of inappropriate behavior. I look at my dog as an example. He has never been abused, he was used in medical research and not exposed to anything for two years of his life. If I approach him "wrong" (loom over him) he will cower, if I take him to a new environment he will cower. I have never abused this dog, his reaction is a "fear" reaction; however, it stems from lack of experience. Which is less fear and more insecurity. At the end of the day we don't get to know what they are thinking and its dangerous to start ascribing emotions to them.