A zebroid is the offspring of any cross between a zebra and any other equine to create a hybrid. In most cases, the sire is a zebra stallion. Offspring of a donkey sire and zebra dam are called a donkra and offspring of a horse sire and a zebra dam called a hebra do exist, but are rare and are usually sterile. Zebroids have been bred since the 19th century. Charles Darwin noted several zebra hybrids in his works.