I use the cheap Walmart brand of ACV. I feed a little less than 1 cup per-day to each of the 8 horses in my care (4 are mine, 4 boarded). It aids in digestion, encourages them to drink more water, masks supplements, and seems to help with flies, a little... It also seems to help the boys stay "cleaner" and need less frequent sheath cleaning.
Corn or Veggie oil has no real benefit, except for fat calories. It is high in Omega 6 fatty acids, which horses already get plenty of from hay (yes, hay has Omega 6's in it). If you want to improve skin/coat/feet, I have had better luck with flax. I feed flax meal from ADM. My feed store carries it, $30 for 20 lbs. Omega Horseshine and horsetech.com's Nutra Flax are very good too, but higher priced at around $45 for 20 lbs. You can also buy (or order) whole flax from most feed stores. You can feed it whole or grind it yourself.