Yes, some horses take cold weather better than others and they do acclimate. I think some breeds take the cold better than others. Babies, older, or sickly horses are obviously at greater risk in cold weather. Two of my horses have been in Indiana over 2 years, but both were born and raised in Florida. Neither of these horses have a winter coat like the rest of the horses. My Florida born horses both shiver before any of the other horses. I do not believe they should shiver. It just can not be good for them. They have plenty of hay and water and are fed grain so they are not shivering because of a lack of fuel. They have had time – 2 years – to acclimate, but they are still very cold natured. I am not an expert in this, but I would say genetics played a part in their fine coat!
The summer can be just as brutal on a horse. A few years ago was my first experience with a bald faced horse and a gray (white from age) horse. Both sunburned easily. The flies were very vicious and both of these horses would have bloody areas where the flies just ate them up. We used sunblock and zinc oxide, fly sprays…. The bald faced horse stayed out all night with the mares because he was a gelding but the gray stallion was a rescue and had been a breeding stallion. I did not trust him to stay in so he was paddocked early mornings and evenings.