Using the Tolkien depiction of Wargs (horse-sized and fairly intelligent large lupines that can be domesticated), where would they likely find an economic niche as livestock or work animals? I would imagine the polar regions, where sled dogs are already popular pack animals, would be a good place for domesticated Wargs, but are there any other environments where Wargs will be preferred over regular horses, mules, or oxen as work animals?
Plant eaters like horses and cows often have to constantly travel to make sure they dont deplete their resources, and occasionally run away when a predator tries to catch them. Their food doesnt run away, which is good.
Predators on the other hand have a tough life. Their food makes them work hard to catch it, and that work costs energy. Other than what we see on television the actual catch rate isnt that high, there's a high likelyhood of the prey escaping. Combine that with the sneaking to find prey, having to cross a larger distance towards the prey and then needing to be strong enough to kill the prey and you can see why predators spend most of the day lazying around: it allows them to conserve energy for their next hunt.
Wargs wouldnt be ploughing your fields any time soon. They are designed for short bursts of aggression with long pauzes and walking inbetween. So any job you give them needs to fill that niche.
Obviously they can be ideal animals to hunt with. The combination of human ingenuity and the animal's natural potential would be powerful.
Guard duty would fit reasonably well, but it depends on your goal. You usually do not want your guard animals to kill and eat intruders, especially if the intruder could be an employee called in to work late or a last-minute replacement to fill in for a sick guy. We teach our guard dogs to disable the intruder, not go for killing blows.
They could function as a pack animal if they carry something light. This allows them to simultaneously guard the object and carry something, although likely not much.
Some jobs that require a lot of sudden force could benefit from having a Warg perform it. Like a short pull on a pulley system. Perhaps on a fishing boat to haul in the nets for example.
Maybe you can find a job for their claws and teeth. With some mouth gear a blacksmith should be able to find a few uses for their undoubtedly immense jaw strength. Those sharp nails and powerful paws could perhaps be used to do simple things like turn wood into chips ready for a good fire.
Your problem is feeding them. As riding animals unless they can be trained to eat dried, salted/processed/fish and grains etc Wargs are unsustainable in large numbers. In small numbers perhaps the problem is not so significant. A rich noble could probably afford to feed a 'stable' of say 12 Wargs. Anyone else?.
A quick search says wolves need about about 4 pounds of meat per day to prosper. On average say 10-20 pounds every few days. If we assume Wargs need at least four times that amount to sustain their greater body mass and do useful 'work' for their owners (and they probably need much more) then the economic cost of using them as mounts outweighs their usefulness.
Your talking about 16 pounds of raw meat a day x 12 equals about 180 pounds. That's at least a cow a week, probably more just for your nobles private stable!
No kingdom or army on Earth could afford to sustain Wargs as mounts except for a small elite minority. An army mustering say 10 thousand riders on horseback can, with some oats and hay to supplement their diet sustain their mounts in the field for weeks and months at a time under relatively normal conditions. An army mounted on 10,000 Wargs would need what? a thousand cows a week! It wouldn't be an army it would be a cattle drive! And any country trying to sustain large numbers of Wargs would end up starving its own population.
If their water need is lower than that for horses, mules or oxen they might be a valid alternative in all the arid and semi arid climates, where supplying water can be difficult.
Even without that they could be viable in any farming location, since they would take care of rodents and other critters feeding on harvests: they would be, at least partially, fed for free and doing so would indirectly increase their master's supplies.
Forget about rural work. These things have a role in security.
You don't need to pay a guy with a flashlight and a stick o guard your museum, factory or mall if a horse-sized wolf will do a much better job for spg food + some bonus human flesh when they catch a trespasser.
Wargs would also be great at dispersing riots. But where they really would excel is sniffing drugs in airports. You'd no longer need to do cavity searches, drug mules would evacuate the drugs off out of pure fear upon receiving a dog greeting from a warg.
I absolutely love the "sleg warg" idea, though that's more something for a fantasy/medieval setting.
How about pest/livestock control in forests? Usually (at least in Germany) huntsmen sit on their high seat somewhere in the forest and wait for the occasional hog or doe to wander by - but a warg could flush them out or even bring them down on its own. This is done in order to reduce damages done to trees/ecosystems by the livestock while their original predators (wolves, sometimes lynx) are not present.
Or they could be used as really big sheperd dogs. No sane wolf would go near a herd guarded by it's big cousin.