Aristotle discusses in his zoological writings, especially the Generation of Animals (ii.8), which animal species in general can interbreed and generate animal hybrids as offspring. Most of them can again procreate themselves (second-order hybrids), with the mule being the hallmark exception to this rule, which Aristotle elucidates at length. This discussion found its way by translations also into Arabic zoology and influenced several authors, among them especially Ibn Sīnā, but also al-Jāḥiẓ. From a philosophical perspective, this question is also connected with the issue of eternity of the animal species and possible origin of new species, such as by way of evolution. A similar consideration is likewise found in the Book on Animals (K. al-Ḥayawān [ed. al-Ḥarbī, Baghdad. 2008]) of the zoological philosopher Ibn Abī l-Ashʿath (d. 975 CE). However, it is, as the paper will show, less influenced by Aristotle, but, rather, by Galen, upon whom he also composed some commentaries. For even though the Aristotelian mule example as that of an infertile hybrid offspring is included, the argumentation is different, as recourse is taken to Galenic temperaments for explanation. Also the Galenic centaur example is discussed, where, besides temperaments, a “natural aptness” is employed as argument for its non-generation. Due to a lesser leaning on Aristotle, the initial question which animal species can interbreed, can only be answered with some incertainties. For, as the Aristotelian requirements for the parent animals such as sharing in a genus, nearness in species, and similarity in both duration of gestation as well as bodily size are either not upheld at all or at least not explicitly mentioned, Galenic temperaments take over this causal role. The results arrived at, though, may be similar, the paper will argue. The paper will finally address whether the minor influence by Aristotle in comparison to Galen necessarily has to be explained by still a direct reception (the Arabic translation of Aristotle’s biological works would have temporally be available to him) or, as will be argued for, rather by assumption of an indirect reception by way of Galen only.