The definition of a species is tricky because of partial fertility. In the old days most ideas of species were just based on what organisms looked like. But as natural historians became more scientific we needed a more precise definition. A popular one is the biological species concept - a species is a group of organisms that can interbreed to produce fertile offspring.
That's fine, except fertility is not binary. Horses and donkeys are an example. They are usually considered different species because although they can interbreed and produce mules, the mules cannot reproduce. Except they can, rarely. So are horses and donkeys really different species?
Or take lions and tigers. We normally think of them as different species, but they can interbreed quite happily in captivity and produce quite fertile tions and ligers. The only reason they don't in nature is because their habitats are well separated. So are lions and tigers the same species?
The truth is, the species concept is fuzzy and trying to draw human lines across nature that is actually blurred is not straightforward.