Did something attack my goat?

Horses are not real slobbery. Since he’s black is there any way you could easily, without stressing him out too much and make sure it’s not blood maybe? Any way a neighbor dog got in? Although I would imagine there would be some major bites if a dog. It’s kinda strange.
I would go ahead and get a temp on him. Check him all over for any bites or swelling. If he is having a rough time of it and seems to be in pain you can give him some banamine, but after you get a temp since it tends to drop their temp some. If he is even slightly sub (under 101) I would pass on it for now.

There is a chance your horses could of been grooming him. My stallion used to do that to one of my barn cats all the time. You just have to be careful that they don't get too rough. My stud got to the point he was picking the cat up by the scruff and putting him on top of the round bale to better reach him for cleaning. The barn cat didn't care but it worried me. So I started putting cat treats leading him away from the studs hay run in to help limit the picking up. It stopped.

What’s weird is that I have two other goats with him and they don’t seem to be harmed in any way

Do you think he could have been just trying to lay up in the shade to chew his cud? Is he acting normal other than the dry stuff on him?
Since it’s on top of his neck, I’d say it could have been the horses. Maybe the female horse thought she should mother him some 😅
If it were dogs I’d think it’d be more serious injury and you’d know something was wrong.
I’d just watch the way he interacts with the horses and see if he seems upset from this incident or anything.

He’s now foaming something white from his mouth other than that he is eating normally but has less energy.

Has he been eating anything unusual? Is he a bit bloated? Do the horses have anything he could of also gotten into?

If you suspect he may of got into something toxic, give charcoal.

Is he bloated?

Is his eyes darting, does he have slack jaw and one side of the face droopy?

A horse could of licked him there on his neck if no injuries.

I have seen horses grab a goat and fling them if they are annoyed… get a temp start probiotic and b complex. The others may have respected the horses space -and not been a target