Start of upset/fear? Depends on the individual horse
The first indication with my horse is if he swishes his tail.
Warmblood at the barn prefers a more direct indication and will give you a half-buck.
The kid pony at the barn, well we haven't managed to find anything that upsets him but fear? He likes to indicate that by teleporting 50ft to the right and taking off at a dead run. Thank God there is basically nothing that he's scared of!
If it is fear that you are talking about, the first sign of fear in my horses is a raise of ears , head and a snorting sound. This occurs when I am riding them, a calm, reassurring voice and a pat with a tongue click and they go on. If you referring to something that kind of jumps out at them or sudden spook, mine will come to a quick stop and wait for me to give them the cue to go on. I am not sure what the question is.......
Ian: Ok, presuming that your horse is like most horses when unafraid (is breathing nicely, has a soft eye, doesn't cause his ears to go rigid, doesn't crank his tail or raise his head to giraffe height, etc.)
The question is: What is the first sign your horse gives that he's scared?
To my mind, it's a very important question, both to help the horse before a spark becomes a wildfire, & to stay safe. I'm sure that I'm not the only one who's gone off to one side when a horse spun to the other, SEEMINGLY without prior sign that the horse had left "ok-land".