"One day, Plato defined humankind as the two-legged animal without feathers. The next day, they say, Diogenes dropped by the Academy with a plucked chicken."
Centuries ago, no one thought that there was a problem. To decide what an "antelope" was, you went out and caught a handsome one, and stuffed it. From then on, everything that looked like that was an antelope. If you caught one that looked a bit different, well, you had caught a defective one.
But typology doesn't work for cabbage, Brussels sprouts and kale, all of which can be easily crossbred with each other. These are interfertile because they (and cauliflower and broccoli) were all bred by humans from the weed known as "mustard".
Clearly, we need a definition based on reproduction, not looks. We could say: a species is group of creatures that breed together. But that doesn't deal with creatures that breed unsuccessfully, creatures that breed if painted, and creatures that breed only in zoos.
Another definition is that two groups of creatures are the same species if, in the wild, there is significant gene flow between the two gene pools. This wording pretty well fixes all of problems listed above. But does it address the question of ring species? And let's not even talk about bacteria and viruses.