Hey there, my 9 yr old mare has about 4-5 on her back and I too have been looking up on this. The most likely things they are, are cysts (a sac of air, liquid, or semi-solids that can form in the body, no need to worry :) ) or they can also be inverted hairs caused by saddle pads not being washed frequently enough.

Hope this help :)

my horse has exactly the same! they dont bother him or affect him in any way other than looking ugly! ive had a back specialist, saddle fitter and two vets look at them! n no 1 seems to know what they are without sending off a biopsy which i dont fancy of his spine! but they bascially said if his not broken dont fix him!

other thing surgested that they might b are fly bites that have turned into cyst! im pretty clueless and have given up on figuring out what they are! his happy and healthy and in no pain so im not worried! x x

hi my horse sometimes gets a couple of hard pea sized lumps on her back or shoulder.  They seem to grow in size until the size of a pea then reduce and disappear.  The vet said they were collagen deposits and nothing to worry about as they were obviously not causing her pain.  It sounds like the same as your horse.  The vet said some horses just get them so dont worry,