Who discovered Canis Major?

No one discovered it . As a stellar constellation it has been known from antiquity. They were all named by the Ancients, who saw patterns/pictures in the night sky, and gave names to these patterns/pictures.

'The name ' Canis Major' , from Latin, means the 'Great/Big Dog'. There is also a 'Canis Minor' or 'Little Dog'.

The star 'Sirius' ,which is also known as the 'Dog Star' has a partner star with it, known as the 'Pup'.

NB

Ursa Major ' Great Bear

Ursa Minor ' Little Bear.