Who discovered the constellations?
Constellations are areas of the sky based around groups of stars as seen from Earth. They aren't physical objects or places, they're just a system that we use to identify parts of the night sky. Therefore, no one really "discovered" constellations. The question should be something like "Who first recognized groups of stars and gave them names?".
We don't know the answer to that question because it happened so long ago. We do know that people have used names for constellations for thousands of years. One of the earliest suspected depictions of a constellation is a cave painting in Lascaux, France, that dates back 17,000 years.
From about 1,000 BCE, names for constellations were in common use. Ancient Greeks listed up to about 50 constellations, and these form the basis of the constellations used by astronomers today.
In 1928 the International Astronomical Union formally named the 88 constellations we now use in astronomy.