A healthy horse should not be fat or too thin. Until you gain experience it can sometimes be hard to tell however once you learn what to look out for it gets much easier.
The neck should not be overly ‘cresty’ which means that the top side of the neck should not be bulging, lumpy and hard. Stallions and some breeds of horses such as Andalusians, Friesians and similar breeds naturally have a more cresty neck than breeds such as Thoroughbreds and Standardbreds (although even these breeds sometimes have individuals with a naturally cresty neck). Pony breeds such as Shetlands and Welsh Mountains also have a naturally cresty neck. It is important to know what is normal for a particular horse and what is a build-up of too much fatty tissue. It is also necessary to be aware of what is ‘normal’ for your breed of horse, some breeds look bulkier than others. You need to regularly feel the neck of a cresty horse to make sure that it is not becoming too hard/lumpy. Hard/lumpy means that the horse is storing too much fat (in much the same way that some overweight people store fat at the base of the neck).
You should be able to run your fingers across the sides of a horse and feel the ribs. Being able to see the last couple of ribs (nearer the ‘hips’) is fine also. If you can see all of the ribs then the horse is underweight (sometimes very fit horses such as racehorses and endurance horses have little or no fat over the ribs but have good muscle tone in other areas of the body which is fine).
If a horse is too fat the back will be very flat or there may even be a groove down the middle of the back as fat on either side of the backbone becomes raised higher than the spine. If a horse is too thin the spine will stand out and there will be a hollow area along either side of the spine.
A fat horse may have lumpy fat pads just in front of the hips and the backside of a fat horse will usually have a groove in the middle (above the tail) and will have pads of fat on either side of the tail head (where the tail meets the body) and down the backs of the buttocks. The backside of a thin horse will be hollow and the tail head will stand out.
In conclusion horses tend to store fat (that can be seen from the outside of the body) on the top side of the neck, on either side of the spine and across the ribs, on the ‘hips’ (i.e. just in front of the pelvis), across the top of the rump, around the tail head and down the back of the buttocks. A fat horse will usually also have fat around the sheath (in a male) or fat around the udder (in a female).
It is important to be able to distinguish fat from muscle. Fat tends to be lumpy, muscle is smoother. A fat horse will tire more quickly than a muscular horse. A thin horse will not only have used up its fat reserves but will often have very little muscle as well (and will also tire quickly). When a body is starved fat is used up first of all and then muscle is used in order to keep the animal alive. When a horse has reached this stage it needs experienced care in order to recover.
Being overweight is particularly dangerous for horses because they can develop a condition called laminitis and other obesity related disorders. Laminitis can be fatal for a horse and needs expert veterinary care.
Good horse care involves monitoring your horse’s ‘condition’ so that it does not get too fat or too thin. Horses can vary in terms of where they tend to build up fat so you need to learn about what a fat horse and what a thin horse looks like so that you can make informed decisions about your horse.
If you are in any doubt if your horse is in appropriate body condition, discuss this with their veterinarian at their next visit. The vet will be able to make an unbiased assessment and make appropriate dietary recommendations.
For more information please see: https://www.equiculture.net/responsiblehorsecare