x1=xi1=b2 is a horse (genus “Equus”) with natural stripes x2=d1=b1; x1=xi1 is a zebra (common English usage).
This term is used to distinguish horses which do have stripes from horses which don’t. The term does not refer to a species. See also: stripe (sriba'a), horse (xirma), mountain zebra (xirnzebra), plains zebra (xirnku'aga) and Grévy’s zebra (xirngrevi).