This question arises given the definition of father and mother as seen in the Spanish dictionary composed by the Royal Spanish Academy. Basically it says that a father/mother is a man/woman or a male/female animal that has engendered/given birth to an individual of the same species (Spanish: "especie").
In the case of mules, they are the offspring of a male donkey and a female horse. A male donkey is considered to be of the species Equus asinus, while a female horse is an Equus caballus. The mule is an individual of the species Equus asinus♂ × Equus caballus♀, but what is exactly that? It is the set resulting from the addition of the two parent species, or is it a complete, new species? In other words, are mules considered to be of the same species of a donkey or a horse? Or what they all have in common is just the genus?