Right now, my fortress has a huge horse population, somewhere around thirty-five or forty adults, all descended from the original wagon-pullers. This is very good, as it allows me to create extra meat whenever I need it, as long as a butcher is on hand (I have four butchers and four butcher's shops, so this tends to work out well). Basically, I've changed to using plants only as the raw materials for alcoholic beverages (the dwarves also seem to really love plump helmet as a snack, though). However, I think the system could be greatly improved.
1. Corrals and stables. Putting horses in cages doesn't seem to make a lot of sense; a simple wooden or stone enclosure we could make would work better. They also seem to hate being shut up in a crowded room behind a closed door; right after I got every horse into the room (by bringing them to the cage inside and then letting them all out again) and someone came in to butcher one, the entire herd escaped. There should be a way to make them all stay in one place and out of the way of working dwarves.
2. A "herder" skill, allowing a dwarf to lead an animal more quickly, and possibly improving other aspects of animal husbandry.
3. A "livestock" designation for animals, which you can toggle in the same screen you can currently toggle an animal's pet availability and whether they can be butchered. An animal marked "livestock" is automatically herded to the corral by someone with the Herding skill, and the Butcher's Shop can have a task to slaughter the nearest livestock animal, the keyword being "nearest" (my butchers often walk halfway across the fortress to haul the specific horse I marked for slaughter in the Animals menu). Livestock cats can never adopt anyone.
4. Manure, as a fertilizer.
5. Sheepdogs, or horsedogs, or gorilladogs, or gianttoaddogs, or whatever kind of dogs you'd use to help herd whatever variety of tame beast you're using as livestock.