Feeding horses is only as complicated as you make it. Horses are foragers, so require a diet consisting mostly of forages.
Concentrates consist of : grains, commercial mixes, and ration balancers. Forages consist of: hay, pasture, hay cubes or pellets and beet pulp (beet pulp is a little bit of a feeding oddity, as it has qualities of both grains and forages, it has more energy than most forages and more fiber than most grains, but it is digested much like forages are, so very safe to feed, but it is not to exceed 40% of the diet). Beet pulp typically comes in two forms: shredded or pelleted; it is recommended that pelleted beet pulp be soaked to reduce the chance of choke, especially with horse that bolt their feed.
Most horses require little or no grain, especially minis, unless they have special needs; growth, pregnancy/lactation, work, seniors, etc. A good vitamin/supplement or ration balancer to balance the forage provided, salt (block or loose) and free choice water is basically all the average horse needs.
The basis for all equine diets should be good, clean hay. They don't necessarily need the richest hay available, just good clean, uncontaminated hay (my preferance is good grass hay). Contaminates consist of: mold, noxious weeds...
All feeds should be fed by weight, not a cup of this or a scoop of that. Horses require 2-3% of their bodyweight in feed per day, at least 50% should be good quality forage. For example, a 250# mini will require 5-7 pounds of feed per day (concentrates and forage).
To fairly accurately estimate my horse's weights, I use a formula developed by Texas A&M:
[heart girth in inches (squared) x length in inches] divided by 330 = body weight in pounds (use 280 for weanling).
Hearth girth is measured around the horses barrel with the tape measure (seamstress tape) placed at the last mane hairs on their withers. Length is measured from the point of the shoulder to the middle of the muscle crease at the rear of the hindquarters.
There are many commercial feeds available for all horse, one of these may just be the perfect feed for your equine; youth/growth formulas, senior formulas, active formulas, pleasure horse and more. They come in a variety of forms: pelleted, extruded (look like dog food, easy to digest), textured, grain mixes and more. You may wish to try the "ration balancer", basically a vitamin/mineral supplement with quality protein, designed to compliment the forage fed.
Above all be sure that your horse has clean water available at all times. In the winter, this may require chopping ice or fitting your tank with an equine safe tank heater.
I'm not a vet or an equine nutritionist, this is just basic information I've learned over 20 years of owning and caring for my own horses. When in doubt consult your vet.