Centromeric shifts often follow Robertsonian fusions. Rachel O'Neill is one of the leaders in this field -- she studies the phenomenon in marsupials (where it has been associated with speciation). Also, it appears that hybridization between different karyotypes can induce centromere relocation in marsupials. Don't know much about horses, though.

I, actually, don't think aneuploidy is that big of a deal. I think the rearrangements suppress genetic flux between karyotypes allowing for the accumulation of genetic isolating factors on rearranged chromosomes. This is the case in Drosophila, and has not been rejected in mammals.

I don't know anything about it, but it makes one wonder if this changes the type of new genetic combinations that can occur during meiosis due to crossing over if there is some minimum distance from the centromere at which crossing over can occur.