In the Categories Aristotle defined substance as that which is neither predicable of nor in another. In saying that a substance is not predicable of another, Aristotle meant to exclude genera and species from the category substance. Aman is a substance but not man. In saying that a substance is not in another, Aristotle meant to exclude property particulars from the category. A man is a substance, not his color.
The Categories treats substances as simples. Though a particular substance, Bucephalus the horse, has parts, it is nevertheless a single entity in the category substance and, hence, incomplex in the way a black thing or a running man are not. Black things and runners are complex because they are aggregates of substances and property particulars. Even if a horse is one substance and, thus, an entity (unlike a substance cum some of its attributes or a group of related substances), a horse is made of parts and one may wonder how it is related to its parts, as well as how its being made of parts coheres with the definition of substance given in the Categories.