Yes, that!! There are horse, ponies & minis that all have metabolic differences - in EVERY breed.
Sometimes those differences run in bloodlines - parent to offspring. We have had some in each of the breeds we have worked w/ or owned over the years. Metabolic issues - both in general & from genetic mutations - seem to be increasing exponentially the longer i'm around equine...
Pasture these days arn't the same either. Everyone wants pretty & uniform & horses are missing out on nutrients contained in different grasses, herbs & even weeds. Supplements & concentrates (grains/feeds out of bags) dont always make up for them...
Horses that are used to being stalled, suddenly being put out on pasture do not always do well & vice versa. It took me several months to get a pony stallion i purchased from TX to eat here in NC. You can imagine, he wasted away to almost nothing. I ordered hay delivered from same source as he had in TX, found a feed store over 100 miles away that carried the feed he'd been getting ( i did get 50# when i purchased him, but as i added ours, he ate less, till he refused to eat at all. The change? His paddock in TX was about 16' wide by 60' long. He lived by himself & wasnt really near any other ponies or horses. His pen was pretty secluded. It did have a little barn w/ completely enclosed 12x12 stall. He was turned out - always alone. Understand ive both owned and worked around stallions. Ive always been happier, & usually they are too, when they have some sort of pasture buddy. Even if not equine. That fella did eventually start eating & did settle into our routines. He eventually had a nice harem of mares that he ran with. He even got started driving & was used to help around the 8 acre farmette we had... but it took a long time (several years) & some expensive & extreme measures... He was great w/ beginner handlers & children in contained areas - again it would be years before he could be safely handled around other equine - he simply lost his mind at first.
Grasses after a hard frost - both in the fall & in spring after pasture grass started growing can be poisonous - some just have huge changes in sugars. Grass during a drought changes too & can make more than just horses sick, but horses can have severe bouts of colic, laminitis, founder & many other issues affected by frosts & droughts.
Big bare spots can be fixed. Pastures can be rotated, "paddock paradise" can be used. It just takes paying attention to conditions & learning good pasture management. And then Mother Nature can throw huge curve balls!!
There is always something to learn w/ all livestock & living imho.
O, & one last thing to remember when it comes to keeping a horse. Zoning regulations where you live. Some towns & counties won't allow horses at all. Others lay out minimum requirements for land in their jurisdiction & it can be different than what a horse actually needs. There are even regulations now about watering horses & other livestock.
Always remember that horses look for ways to get into trouble, the more expensive, the better...
now, granted, I don’t think it is right to keep a horse on such a tight lot, we have around 2.5 acres and a huge yard, and I still don’t think that’s enough for a horse, but point is, they protected the horse owner. There are lots of other people that have livestock in town too, horse or otherwise. Lots of goats and chickens lately. And of course gardens. So they definitely fully support it here. 
maybe eventually ha no clue what discipline I’d want to try though. I didn’t really have a specific one. Also not sure I’d even be able to find a place that does western too. Most around here do English. 
