A0.15.1284: Playing on a large (300x300) map, training half a dozen animals (pigs and retrievers) to haul was initially invaluable, since there are a lot of long walks to be made.
However, after time, the animals seemed to haul less or stop entirely. The only major change I saw other than normal colony growth in this time was that the pigs were breeding rapidly, so there's 50-70 domesticated animals on the map now, with only 16 of them trained to haul.
After reading other reports of animal hauling being unreliable, (i.e. knowing they should only be expected to work occasionally, not all day) I carefully observed the hauling trained animals, and noticed that for a period of 5 days, none of the hauling animals did ANY hauling, at all. All were not restricted to any area, and the only hauling tasks performed were by pawns. (16 hauling animals, and not one did any hauling!)
I tried creating a new animal area that included only the items to be hauled, and the destination stockpile, and their sleeping area, then assigned all hauling animals to that area. After that, I did observe animals taking some hauling tasks, but only for items that were very close to the fort. They would no longer travel the map to pick up distant items, even ones that I'd placed the new "hauling animal" area over.
I don't have any better observations that might suggest specifics for this bug. I can only report with certainty that there ARE situations where animal hauling fails completely and animals stop working for days at at time. It appears to be intermittent, so maybe pathing or AI choices are involved somehow? Maybe there's some limited resource that is consumed by the many non-hauling animals?
I tried unrestricting all Hauling animals, and watched them do absolutely no hauling for a day, then tried reassigning them to the "hauling" area that included stockpiles and items to haul, and almost immediately 4-5 of them took up hauling tasks, but only for nearby items. They're still ignoring items that require a long walk, that they were perfectly able to haul earlier in the game.
Still trying to figure out what changed to make animals haul so much less frequently. Maybe placing walls messed up pathing and now they can't find their way outside to haul?
Ooh, interestingly, they will haul to a distant stockpile, if they are close to the item to be hauled. Maybe colonists can see much farther away to find a hauling target? I wonder if assigning them all to a colonist, drafting the colonist, and herding them all to a distant location will get them to target items out there?
I still have no idea why they all stopped hauling for a while.
So, dragging the animals out to a pile of steel slag to be hauled definitely works. Several of them grabbed slag and hauled it back as soon as the colonist was undrafted, while they were all standing near the items.
Loaded up the save today and the animals have started hauling from distant locations on their own again! I didn't change anything about the fort layout, areas or stockpiles. Maybe they remember walking in certain areas, or past items eligible to be hauled?
Three things I have noted from various threads over the last while.
Hauling animals will be more likely to haul if the "master" they are assigned to is actively hauling.
Hauling animals will prioritize hauling "nearby" things over "far" things, just as pawns.
That being said, hauling animals appear to "give up" on "searching" for something to haul if there is nothing within a certain radius of their position.
Animals seem to have issues with all pathfinding while "unrestricted" for some reason.
"Restricting" these animals to a map-wide zone appears to fix this.
For some reason.
No idea why.
I've already reported the A15 animal hauling bug here: https://ludeon.com/mantis/view.php?id=2698
The short version of it is that animals will look for the closest thing that theoretically could be hauled (either it's not in a stockpile currently, or it is in a stockpile and a higher priority stockpile that could contain it exists). If the animal is not capable of hauling it to a destination (there is no stockpile that could hold it, or the stockpiles that could hold it/have a higher priority are full or not reachable by the animal), then it will give up and won't haul anything.
My fix mod includes a fix for it: https://ludeon.com/forums/index.php?topic=27267
Edit: from past threads, it seems like this happens to come up frequently with stone chunks that have been marked for hauling.