Ancient Zoan devil Buah allows the user to transform to anceint aka prehistoric beast. X Drake,a supernova and captain of the rookie drake Pirates is the only known ancient zoan right now. He can transform into a bipedal dinosaur, it's not a T-Rex, his arms are too long and he's not tall enough, likely 10-15 ft tall (he stood slightly taller than Kuma-lookalike PX).
Marco of the Whitebeard pirates ate a Mythical Zoan devil fruit, which allows him to transform into a blue, firey phoenix.
While ancient zoans are rare, the mythical zoan is rarer. To the soalan which is lebih powerful, I would say the phoenix. The api, kebakaran allows Marco to heal from any wound instantly.
An Ancient Zoan Type Devil Buah is a Zoan Devil Buah that alows anda to become an ANCIENT animal, like a Dinsaur. For example, X drake has an Ancient Zoan type. He has the T-Rex. However, Marco does not have an ANCIENT Zoan Type, he has a MYTHICAL Zoan Type. Since he is a Phoenix. Phoenixes are not ancient because, as far as we know, they never existed. So, it's MYTHICAL because they're a MYTH.