There is literally an endless supply of different horse & equine organizations out there. Sorting & listing them is an enormous task and one I’m really glad others have already accomplished. I see no sense in reinventing the wheel.
At the risk of sounding redundant, this is a list of horse organization lists – there are so many out there already that everyone should be able to find what they are looking for. I think I’ve covered all of the basics and I expect that this list will continue to grow over time. Be sure to leave your own list or a any great ones you know of in the comments!