There are two different types of isolating mechanisms
Pre-zygotic: Before birth of the offspring
Post-zygotic: After birth of the offspring
Habitat isolation: when species are incompatible due to location, or time of fertility.
Behavioral isolation: when species are incompatible due to differences in mating calls, or behavior.
Sexual/mechanical isolation: when the species are unable to sexually copulate due to the incompatibility of genitalia.
Hybrids: the animal kingdom has many examples of hybrids, where two species can interbreed and produce an offspring. Some of these offspring are infertile, such as the Mule, however there are hybrids that are fertile and able to produce viable offspring.
Howard, D. J., & Berlocher, S. H. (1998). Endless forms: Species and speciation. New York: Oxford University Press.
Matute, D. (n.d). The role of founder effects on the evolution of reproductive isolation. Journal Of Evolutionary Biology, 26(11), 2299-2311.