I'm in mourning right now. One of the horses at my friend's barn, a 23 year old Quarter Horse mare who was retired from giving lessons (due to a soft tissue injury, not her age) dropped dead. Unexpectedly. She was in good weight and otherwise healthy, showing no symptoms of any disease.
Why would something like this happen?
One possibility is toxins. (In this case highly unlikely - she was being very well looked after and not allowed to get into anything she wasn't supposed to). But there was a high profile situation in 2009 when 21 polo ponies dropped dead after being given a vitamin supplement that had been mixed incorrectly - they had a selenium overdose). Toxic plants can also cause sudden death, usually a few hours after the animal ingested the plant.
More likely in this case, though, was an aneurism. As mentioned, horses don't get heart disease the way we do. What can happen, though, is an aortic rupture - a break of the major artery above the heart. If somebody says an old horse died "of a heart attack," then that's most likely what they mean. Older horses can develop a thinning of the arteries that can lead to an aneurism, most likely in the arteries near the heart, but sometimes in the brain (it's rare, but it does happen). This thinning generally shows absolutely no symptoms until one day - the horse just drops dead - often, but not always, during exertion. (In fact, horses can and do drop dead under a rider - sometimes resulting in the injury or death of the rider. Good riders are taught how to get clear if a horse goes down for any reason, but you don't always have time). There's generally no warning and nothing the horse's owners and carers can do.
R.I.P. Cascade...I'm going to go back to being quite upset now. She was a truly special one.