I thought the info would be useful to us here on the HorseForum as well!
I tie to trailer and highline using a bowline with a quick release and I generally use several to discourage nimble mouths!
Bowline instructions - TrailMeister KNOT tying Help
Another thing to think about is the danger of having having a saddle on the horse when it's tied to a highline. I did this ONCE. After I had to cut my favorite lead from around the horn, I decided that it's a good idea to untack before I put the horse on a highline. Luckily no one was injured.
If you're around horses IMHO you should have a knife on you. Giant rambo knifes aren't necessary, just a pocket knife to cut baling twine, open a package, or cut a lead in an emergency!
Also, you should know how to tie a few knots. If you don't know to tie knots useful for horse owners there are a number of places where you can learn to do so. I like this one.
Knot tying instructions / illustrations / video - TrailMeister KNOT Tying Help