Yes, I do 'usually, teach a horse to accept a slicker, by first lunging him with one draped over him, then I get off to put it on, for that first ride, and after that, can put it on without getting off
I skipped that step once on a colt, because he had been so easy to start under saddle, accepting everything
We were out on a ride, and I always have a slicker tied on. It started to rain,, so I just treated that colt like I would one of my broke horses, untying it, and then letting it fall over the sides of my horse, as I stick my arms in,
Well, I got one arm in, and that slicker fell down his one side, and he bucked for the first time. Hubby thought it was rather funny, as I tried to get that arm out and drop that slicker. Luckily, soon as I managed to do so, he quit bucking, I then did the smart thing, got off , ran that slicker over him, once or twice, put it on, got back on, and all was fine!
if you are ever going to lead a pack horse, get your saddle horse used to a rope under his tail-just in case!
Some horses don't mind saddle bags, even for that first ride, but we once took someone on a ride, who used saddle bags on his horse for the first time, and when we trotted, that horse bucked him off
I have also seen a horse bolt , when his rider pulled a plastic bag with sandwiches out of his saddle bag
I keep hold of the reins, first time I open a pop can on a horse