From the perspective of a lifetime horse rider/breeder, rather than a zooarchaeologist, I would suggest this shows a particular breed/type rather than any injury.
 
I would look to a horse in the Anulusian, barb etc linage where tou see 5 lumbar  vertebrae  commonly or a fusion on the 5th & 6th providing a short backed agile animal suitable for mounted combat with strong hindquaters.
 
eg off the web due to lack of refs here !
 
Ocracoke ponies have distinctive physical characteristics: five lumbar vertebrae instead of the six found in other horses, 17 ribs instead of the 18 found in other horses, and a unique shape, posture, color, size, and weight. For more information see the Ocracoke Island section of our Area Overview chapter.
 
http://www.insiders.com/outerbanks/main-attractions5.htm
 
On Spanish Colonial types and the fused lumber see
 
http://groups.msn.com/NEWMBA/whatisahorseoftheamericashorse.msnw and many many other sites
 
I would , look for strong frontal indications (eg chest neck)in a draught horse of that period with the system of harness that was typically believed to be used and developed hindquaters in a ridden, mounted combat horse which was expected to perform fast turns. 
 
I would imagine modern vet refs would cover this fusion in depth as there are many horses for this type of linage active in dressage and many other equine sports and the variation of vertebrae number is fairly commonly recognised by horse owners.
 
Professional horse back practitioners may also be able to give you further insight into this.
 
Hope that is of some use. 
 
Mel
 
 
 
 
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