Horses adapt to their environment by growing thick, shaggier coat to protect them from the harsh and cold weather and especially in the winter and in colder places in thr world and vice versa if the climate is hot then the horses coat will thin out more than usual so they don't over heat. Also they will adapt around you and at what time that you feed them ride them, wash them and ride them etc. Hope this will help you. :)
They adapt by changing, so like a horse in the winter grows a 'Winter Coat' , so that means it adapts to suit their habitat.