As mentioned above, the Akaviri versions may have been the ones that actually served the empire. It was the red variety of those. However, here's some sources:
"After the Serpent-Folk ate all the Men, they tried to eat all the Dragons. They managed to enslave the Red Dragons, but the black ones had fled to (then) Po Tun.
A great war was raged, which left both the cats and the snakes weak, and the Dragons all dead. Since that time the cat-folk have tried to become the Dragons. Tosh Raka is the first to succeed. He is the largest Dragon in the world, orange and black, and he has very many new ideas."
From Mysterious Akavir. Although the second to last sentence may imply that other Ka Po' Tun cat-people succeeded in becoming dragons, and that these are the ones we see in Skyrim. But then, what's the relationship between Alduin's return and the return of dragons?