Here's a remark I made about setting boundaries in a completely different forum:

More from the pasture: The herd has settled. The chestnut mare is boss, and she's a good alpha. My paint mare is second. The old draft horse gelding is third, and the grey is fourth. Today I went into the pasture to catch the paint. I call her, and she comes to me. But chestnut boss comes up and herds the paint away from me. So I have to have a talk with the boss.

I make eye contact with her, and walk up to her. She glares at me, but doesn't show hostility. I wave my hands a little at waist level, and she backs up one step. That's a mild submission, and all I needed. Now I can halter the paint mare. The boss mare watches closely, but does not interfere.

As someone else says, handling horses will teach you about setting boundaries in a very practical way.