I know it sounds heartless, but the Vet probably didn't give Gus pain meds at first because he knows that a hurting horse is more likely to stand still, and not feel frisky enough to start bouncing around and damage himself. "Stall rest" is actually very stressful for a horse; some really high strung horses have to be kept sedated to tolerate it. Also, there's the "catch 22" of ulcers. A pain-stressed horse is at risk for ulcers, but the medications that are used for pain in horses also cause ulcers. If you give pain meds long term, you usually have to give anti-ulcer medications as well.
In the short term, as long as the horse is eating well and acting reasonably normal, pain meds are often considered more for the owner's comfort than the horse's.
Em, be very careful, ok? You have described this guy as a wild horse - he isn't. He's something even more dangerous - an untrained horse that is
very used to people, but which hasn't been taught to respect them. It was obvious at your first meeting. A wild horse knows people for the predators that we are, and keeps his distance. When you drove up to the pasture, a "wild" horse wouldn't have let you get within 50 feet of it. A properly trained horse would have approached to a respectful distance, and then waited quietly and let
you come to
him - that's respect in a horse. This guy walked up to you, and he didn't know you from Adam. He was saying, "Hello, this is my space, what are you doing here?" Not exactly hostile, but not quite "Hi, buddy, so glad to see you" either. You know that the immediately previous owner did nothing at all with the horse, but does anybody know about the owner(s) before that? This guy has clearly been handled enough to have positive associations with humans prior to your meeting him, even if it wasn't formal training.
A horse in a new situation starts finding his place in the pecking order immediately. He wants to know who he can push around, and who can push him around. When I have a new horse come in at the barn, I can expect all kinds of space invasions and other acts of disrespect until I establish the fact that I am the "dominant animal," as even well-trained horses don't automatically accept that a human is the boss. There are many subtle ways that horses can test the boundaries, especially when it comes to food. There's a saying, "There is no 'free time' with horses. Every minute you are around one, you are teaching him something. Whether you are teaching him good things or bad is up to you." People may think they are paying a trainer to teach the horse, but what the trainer does is teach the horse how to respond to him. You teach the horse how to respond to you, every day. Gus is a draft, so he's probably always going to be low-key, but drafts can be pretty pushy, too. Love him, he deserves it, but don't let him get away with anything, either!