Ppine,
There are responsible horse packers and there are slobs as well. I have worked in areas in Washington, California, Utah, Colorado and Idaho where horse packers have left piles of cans, empty bottles, deer and elk heads and innards, feed sacks and sheet plastic behind for others to clean up. To be fair, I have also seen the same sort of thing done by backpackers but usually on a much smaller scale. The important difference, in my mind, is that horse packers can carry more stuff in to be left behind and they can carry it in further than do most slobby backpackers. And, during a 40 year career in resource management, I have seen far more damage done to fragile mountain areas by horses and their packers than by backpackers. Again, there are responsible packers and there are the slobs. I'm talking here about the slovenly, lazy 10% not the folks who do volunteer trail work and search and rescue support.