When riding a horse and/or donkey, chunks will not load after a certain distance until you get off the horse. from what I've experienced it happens after about 8 chunks. you must get off the horse for the chunks to load, and sometimes they still won't load. Pressing F3+A doesn't seem to fix it either. After reaching the chunk limit your character freezes and you can't get off your horse, have to save and exit to menu to fix it. I reported this in 15w45a, and the bug tracker said it was resolved, but it's still in 15w46a.