
It's not that he can't die, rather having longer life span than any other, almost eternal.
(He did say "I'm not that old!" When the Rooster called him gramps (≧∀≦)
Assuming that, he still got a long life ahead of him)
So yes he was 1 of the 12 Zodiac leaders of the 1st race that starts it all. While the Dragon lives on 'cos of his long lifespan the rest of the Gods their offspring/heir will take their place when it's time for their eternal sleep.
(Kinda sad seeing his beloved once passing away before him over and over again for centuries ╥﹏╥)

Yes, all animals except for the dragon have a limited lifespan so the 'original 12 animals' refers to the first generation that participated in the race
Correct me if I'm wrong. When they said the original 12 zodiac it meant that the dragon was the only animal that originally participated in the race and since he is a mythical creature he does not die thus has no replacement unlike the other animals that would die and the zodiac spirit would find a new replacement? If I'm right than it make sense about the phrase "original 12 zodiac". If not can someone explain it to me?