My horse as a kid was a nervous sweater, she would have neck sweat ten minutes into a ride just walking and we worked up a sweat galloping a lot. She lived in a very clean stall. She was a mostly white pinto and I did wash her a bit. She always had a slightly acrid sweat smell, rather than a horsey smell. She adored being brushed and scratched.
My current mare lives in a runout stall and is quite clever about pooping and peeing outside and sleeping on shavings in her very clean stall. She’s a Paint with good markings but probably 75% chestnut. I rinse her off in summer, she has a very shiny coat that repels a lot of staining even on her white spots. She doesn’t sweat a lot unless it’s hot out and we’re moving fast or long distances. I feel like she has a very mild dry horse odor, often very little if it’s summer and lots of hosing off. She prefers that to being brushed!
Both horses were very healthy, not prone to skin funk, and shiny.
I feel like horses on field board smell good, they roll in the grass and get rained on and never sleep in their own poop. When I notice a horse with a stronger odor I tend to associate it with hygiene, sleeping in a dirty stall or with dirty blankets. I haven’t been around enough horses with health conditions to see if they inherently smell different. I do think hygiene is a bit different from inherent odor.
I will say that fresh manure can smell very different between different horses, some are very sulphuric, some are fetid, smell it right down the aisle, but I don’t know enough about everyone’s feeding, supplements or health and gut issues to know why.