Edit: just read the article. It was a Zebra stallion crossed with a horse mare, not the other way round as stated in the original post.
A Zorse is when a Zebra stallion impregnates a horse mare.
A Zorse can not be produced by covering a Zebra mare with a horse stallion, it has something to do with their chromosomes. A horse has 66; a Zebra has 44. In order to produce offspring, the smaller number of chromosomes needs to be on the stallion's side; therefore the sire must always be a Zebra stallion. Any attempts to put a horse stallion over a Zebra mare would never result in a pregnancy.