When the war started, motor card and vans still shared the roads with horses, so this was also the case in support of troops. Although awareness about cruelty to horses was rising, people were still fairly indifferent to whether they suffered or not as they were still seen pretty much as cars and vans are today - they just ate hay and oats instead of petrol. It was commonplace for horses to drop dead in the streets, so if any horses got what we would deem reasonable treatment in war service it would be down to their handler's whims rather than any regulations.
There is a book called "War Horses" that carefully researches the use of horses in warfare throughout human history, including WWI. The name of the author escapes me. I have read it, but I could only read only a few pages at a time because of the brutality described within it. It describes the gear, most of it designed to inflict pain. Sometimes, there were paragraphs showing some humane concern about the armies and the horses.
upwards of 8 million were killed in WW1, and they were used as carriers for goods, soldiers, pulling guns, ammunition, most of the info could be find using wikipedia, or if really interested The Imperial War Museum website,