Well, it has to happen some time, either your horse is quite ill and needs the vet, or its hooves need trimming. If you have been vigilant and have handled your horse right from the beginning this does not have to be a traumatic event. I think safety requires that you at least prepare your horse for what is going to happen to it well before the professional turns up.
You need to make sure your horse can be tied up. Go through this exercise almost every day. Sometimes for a few minutes, sometimes up to an hour. It might be boring and your horse may paw the ground. This is not a vice, he is telling you he is bored and it is okay for him to be bored. I find people who try to stop their horses from pawing are a bit mean. What else is there for him to do? You don't have to entertain him while tied up, but you don't have to make it harder on him either. Let him paw. He'll get sick of it soon enough.
You have to make sure you can touch every part of his body and bits without him making a fuss about it. You need to spend time making sure you can do this. Scratching along helps, because it is relaxing. Then make sure you can do it all while he is tied up.
Then you need to be able to lift up each leg, and hold it for a little while without him trying to pull away from you. We use the word "Lift" and we pick up the leg. You can either squeeze the tendon gently or pull the fetlock hairs as you say it. After a while, you will get a leg lift.
Okay, now the next thing to do is to find another person, who the horse does not know well, but is fairly confident with horses, to touch him, and to lift up his legs. This is just to get him used a strange person handling him. It needs to be done till it is not a big deal to him.
If you are able, teach him to put up with either a slap on the neck about where needles go, or if you have a needle, put it into him. But be condifident you know how to use a needle. My vet to the most extent will put the needle in, the attach the syringe and then plunge anything inside the syringe into my horse. But if you have not done it or are not confident, just slapping is the better choice. It doesn't have to be a hard slap, you aren't punishing him, just getting him used to having something going on there.
If you have a cooperative vet and/or farrier, have them come out just to visit the horse so that the horse gets to know the smell of the professional (see smell below). Then it isn't really a stranger.
My mare would go home with our vet it he would let her. She actually nickers to him whenever he comes like a long lost friend. She isn't scared of him at all, even when he walks up to her with a needle in his hand. I don't actually have to tie her up for his visits, just hold her lead. Mostly to stop her from following him when he leaves.
So practice before the vet comes and it wont be nearly as traumatic as if he just rocks up and assaults your horse (in your horse's mind).