I think I know what you mean by "loose" ...
Diarrhea is defined as liquid stool. Can dehydrate your horse and is to be taken as a major red flag that something is really off in regards to your horse's health
Dry stool is defined as stool that breaks apart upon impact with the ground....i.e. It crumbles. It is not a good sign, can signal dehydration, and cause colic like symptoms
Loose is defined as stool that is solid but wet and cannot hold any form. Meaning there is no shape or formed balls...it splats onto the ground and becomes one big pancake.
Usually minor...but a signal to the owner, nonetheless, that something needs to be adjusted or remedied
"perfect" horse stool should have a shiny, wet sheen to it and hold it's form...i.e. It should look like moist poop balls.
I've put alot of effort into the study of horse poop...
Our horses got loose stool when we first put them out on grass in the spring....as soon as it got loose, we cut their grass time down until the stool stabilized and became "normal".
I HIGHLY reccomend you put your horse on a probiotic supplement. Loose stools can be something minor...or not. But it is a signal to YOU that your horse's digestion is off. It is not working as it should. Probiotics can be invaluable in stabilizing the digestive process, especially in the hind gut.
BOTH of my horses are on probiotics, given as a daily supplement. Except for the spring grass incident, their stools are always normal.
Alot of things, minor and not, can upset the normal colonization of the gut....probiotics can help resolve that.