People this isn't fit ...it's like CALL THE HUMANE SOCIETY or County Animal Control.
Source page here for the horse above.
This horse was given a body score of 3.5-4 with the indication that this horse might be an endurance horse, eventing horse. Based on the lack of muscle tone I'd say it needs some FOOD. A new home might work in its best interests too.
Please if I should present anywhere with my horse that thin with an excuse that I ride distance someone please just take my horse home, never look back, and go feed it. That horse is so thin that its body is no longer proportional. I want to "feel ribs" but you should not "see" ribs. That is appalling. If an endurance horse uses its fat layer and stored muscle glycogen to fuel the slow aerobic burn of long distance riding, isn't it reasonable that the horse should have some fuel attached to its body to burn? Emaciation does not fuel the motor!
Even Purina's Body Score chart indicates an athletic horse such as mentioned above could succumb to a score of 4 and be healthy, but they also state the ideal as around 5.
So how do you know if your horse has got too thin? Read over that Purina Mills body score chart (or other) and compare it with the condition of your horse. But honestly, can you not just look at your horse and tell it needs fed a few groceries or if you can't do that, a good fat supplement to increase calories?
I will grant you that Phebes is OBESE and I am at fault for not riding her. Cree is insulin resistant and we struggle balancing his health with not having him too thin. Too fat causes a cascade of life threatening issues for him. His caloric intake has to be low, and he must stay thinner than I prefer to see a horse. Journey is closest to the mark of a body score of 5-6 and will begin to get kind of lean towards the end of the ride season, but we put on the brakes at a body score of less than 5, and have a date with the "fat farm" to put some weight back on.
Here is an article on Nutrition for the Endurance Horse also Feeding the Endurance Horse.
Too Fat too thin or Just Right
Here is one on the disaster of Overweight Horses.
Here is an article on Four Reasons to Feed Fat.
This article gives some reasoning behind Why is This Horse So Skinny? Maybe it is all the exercise, and maybe it is not. I've seen some pretty nicely filled out hundred mile horses, and a few train wrecks of bones.
Personally, I do not want to see a "dip" on either side of the withers, I do not want to visually see all the ribs, a hint of the last one or two won't throw me under the bus. A defined hip is alright if it is covered with toned muscle, the back should not be "hollow" on either side of the spine, I vote for a nice divot right down the center, the tail head should not be prominent, the coat should glow or sparkle if it is a grey, the horse should have a bright eye, and a good appetite. That isn't very scientific of me is it? Though I do understand the entire distance runner/ distance horse body comparison thing, I also understand (via veterinary statements) that the thin endurance horse is at more risk for metabolic failure than the horse in good flesh. So with all things being equal the horse starting out with the better body condition score can safely maintain and fuel the work being asked for longer than his bony counter-part. I'm not saying both horses aren't fit. One just has more fuel in the tank.
Journey's score was lower but probably still a 4-5 when I first got her.
Then I plumped her a bit.
Her body score here last fall (below) was spot on probably a 5-6.
The point of this is not a call to judgement on anyone. We all feel I suppose that we know what is best for our horses, we know how much we ride vs. what we feed. But if your performance is falling short of the goals you've trained or conditioned for as in my horse is running out of gas...and can't finish the ride, perhaps examine if there is enough fuel in the tank and adjust accordingly. Check your horse's body weight score. Has the horse reached is physical limits or has it drawn from and used up all its reserves?