´Hamal bin Malik said:` "The Messenger of Allah ruled that a slave (should be given as Diyah) for a fetus." Tawus said: "A horse would do in place of a slave."
Hadith Referenceسنن نسائي / كتاب القسامة والقود والديات / 4820
In the hadiths, the explanation of "ghurrah" has been given as a slave (male) or a slave-girl (female). Hazrat Tawus included the horse, and some people have also included the mule along with the horse. In some marfu‘ narrations, the mention of the horse and mule is an insertion and is due to a narrator’s error, because when the explanation of "ghurrah" has been given by the Messenger of Allah (sallallahu alayhi wa sallam) himself as a slave (male) or a slave-girl (female), then there remains no need to turn elsewhere. The statement of the Messenger of Allah (sallallahu alayhi wa sallam) is the decisive word.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 4820