Homunculus (not to be confused with the fictional Chicken-Man-hybrids or the Homunculi from Full Metal Alchemist) is a Latin term translated to "Little Man". Their babies are Baby-Adult hybrids that look less pretty than your average teenager, with their first picture found in the Medieval times.
They are the subspecies of humans and the only difference between a Homunculus and a Human lies in with their babies. Human babies look cute, while Homunculus babies look ugly.
The reason why they are extinct is because they look ugly and everyone homunculus adult prefer to adopt a human baby. Little that they know, that makes their population drop all the way down to 0 because they are not reproducing.
Humans all around the world visit various history museums to see pictures of their sub-spicies from the medieval times, other species went to these museums to to learn Human History, but it is for certain that all species roast these pictures.