[gore3000]

How did the various species of horsey's all get different chromosome numbers,

Well they are still the same species. It seems that the number of paired chromosomes in horses varies quite wildly amongst thoroughbreds, but they can mate without problems. Don't know the reason for this but they are the same species though.

[Stultis]

So whole chromosomes can split and join, but a single gene can't be duplicated and still function. Amazing.

BTW, there are a number of different species of Equids. Off the top of my head I believe there are, in addition to the common horse, two or three species of zebras, two or three of asses, and also several species of other wild horses.