From the University of Toronto:
The offspring of two horses will be a horse, and the offspring of two donkeys will be a donkey. But the offspring of a horse and a donkey will be a mule. And most mules are infertile, so mules are not a species.
For decades, this was the basic logic used to define a biological species: a population of animals that can successfully reproduce. But that definition is shifting. There are hybrid animals that produce fertile offspring and grow into larger populations, but are not necessarily their own species.