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Citaat:

Some, though not all, have 5 lumbar vertebrae instead of the usual 6, and 17 rather than 18 pairs of ribs.

Citaat:

Purebred Arabians actually have two less vertebrae than other breeds. The reason they have one less pair of ribs is because there is one less thoracic vertebra for them to attach to. Each pair of ribs attaches at either side of one of the thoracic vertebrae. So, they have a total of 17 thoracic vertebrae and the corresponding 17 pairs of ribs.
They also have five lumbar vertebrae instead of 6 (loin area), which gives them shorter coupling than other breeds. They are known for their short, sturdy, wide, deep loins and ability to carry more weight on their backs for their size than other breeds are.

There are some purebreds who deviate from this norm and have only one less vertebra, but that is not the norm. Arab crosses like your horse don't typically have two less vertebrae, but may have one or even two less.

Arabs also have greater bone density and sturdier hoof quality than other breeds, which also contributes to their superior ability to carry weight. Their rib cages are also wider and more rounded which accommodates larger lung and heart space within the thoracic cavity. This also compensates for the space lost because of the shorter rib cage. This is why Arabian saddle trees are flatter pitched than many full quarter horse trees are.

Also, the shorter rib cage is needed to accommodate the shorter lumbar span in order to allow room for the stifles to come forward without hitting the ribs. Arabs are also physiologically slightly cow hocked for the same reason, as the stifles joints angle out more which brings the bones below the joints to angle medially. This normal conformation in Arabs accommodates the shorter loin and prevents interference of the stifles with the ribs. Even the way they metabolize glucose differs from other breeds, and they are beautifully designed to carry heavier riders over long distances with better energy efficiency than other breeds.

Your horse may have inherited some of these characteristics, and your vet can help you to determine which ones your horse exhibits.
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Add............... There is no way to count all of your horse's ribs as suggested above. Note the position of the scapula and the corresponding situation of the first thoracic ribs................

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Source(s):

Registered Nurse and 58 years with horses (including Arabs)
Study of Equine Anatomy and Physiology