This condition is commonly called high/low, and is more common in Arabians than most breeders care to admit. My Half-Arab is built this way, his front feet don't look like they belong to the same horse. He's 20 and sound on them, thanks to a lifetime of good vets and farriers working together.

If you catch on to this when a young horse is still growing, you can sometimes improve the situation through corrective trimming. One vet I consulted says he has surgically relaxed the tendons on some 'clubby' feet in weanlings and seen good outcomes. But, once the horse is beyond that stage, ie 2 or older, the best thing to do is to trim/shoe each foot so that the bones are correctly aligned, and live with the aesthetic difference. To do this right you need a lateral view x-ray of each foot, and a farrier who knows how to read them. The very best but very expensive way to do it is to have the farrier and a vet with digital x-ray machine there at the same time. X-ray, trim a bit, x-ray, trim, till you've got the right balance. Then measure the heck out of both feet so the farrier can reproduce that balance. Get new x-rays every year or two, because things can change over time, and trim angles may need to be adjusted.