Horse
It can doze standing up because its legs lock, but it has to lie down to reach deep sleep. A horse is a herd animal first and does badly kept alone.
Photo by docentjoyce, CC BY 2.0
Overview
The most important fact about a horse is anatomical: it cannot vomit or regurgitate. Anything that goes in has to go all the way through, so a blockage or a stomach filling with gas or fluid has nowhere to vent, and the stomach can rupture. That is why colic is treated as an emergency in horses and as an upset stomach in almost nothing else.
The rest follows from the same design. A horse is built to eat forage slowly for most of the day, so forage comes first and hard feed only where the workload actually demands it. Laminitis is the other constant risk, and it complicates roughly a quarter of colic cases.
Temperament
A herd animal that reads the room continuously. Horses are prey animals with fast reactions and long memories, and most of what looks like disobedience is a horse telling you it does not trust something yet.
They do badly alone. A single horse in a field is stressed even when it looks settled, and company of some kind, another horse ideally, is part of the basic requirement rather than an extra. They live 25 to 30 years, so buying one is a commitment measured in decades.
Care and grooming
Forage first, water always, and a diet changed slowly rather than suddenly, because the hindgut fermentation that digests grass does not cope with abrupt change.
Feet are the recurring cost: trimming every six to eight weeks, shoes if the work demands them. Teeth need rasping annually. Add vaccination, worming based on egg counts, and the fact that a horse needs looking at twice a day every day of the year, in weather that makes that unpleasant.
Living and exercise
One to two acres, rotated, plus shelter or a stable and somewhere dry to stand in winter. Fencing needs to be visible as well as strong, because a horse at speed will go through wire it did not see.
Exercise is not optional. A horse in a stable without work or turnout develops both physical and behavioural problems, and the standard answer is daily turnout with other horses plus whatever ridden or ground work the animal is fit for.
Something here wrong or out of date?