I admit this is not the best video. I went into my local Apple store and asked if I could zoom with the iPhone and they told me that the software to do that should be coming out in the fall. So I apologize for the long range vision.
What I want you to notice here can still be seen though. Every horse is born with their own distinct thoughts and personality. I do not know why Popcorn has chosen the roll of protector. He is however very interested in keeping everyone at peace, even if that means he must work very hard to achieve it.
Popcorn knows Vaquero. He knows that Vaquero (the bay) was a stallion until this spring. He knows that Vaquero was aggressive and he (Popcorn) is not yet convinced that he will be nice to other horses. So Popcorn keeps the peace by keeping these two separate.
Have you taken the time to study your horses in the pasture together? What behavior do you notice in the pasture and how does that transfer to when you are riding them? Watch for tomorrows video of Popcorn and more explanation.