Ancient Zoans are a subclass of the Zoan type of Devil Fruits in One Piece and they're quite special even among the Zoans. These Devil Fruits allow the user to transform into an ancient animal or their hybrid at will.
As expected, these Devil Fruits are special and they grant an extremely high level of resilience to the user, along with tremendous brute strength and speed. So far, a total of nine Ancient Zoan Devil Fruits have been revealed in One Piece and all of them belong to the crew of one of the Four Emperors of the Sea, Kaido.
9 Kumo Kumo no Mi, Model: Rosamygale Grauvogeli
The Kumo Kumo no Mi, Model: Rosamygale Grauvogeli was eaten by Black Maria. This Devil Fruit is quite powerful and allows her to become a giant spider at will.
As with all the Ancient Zoans, it packs tremendous power and durability and moreover, gives her the ability to trap people in her webs. Afterward, she can digest them with her venom, killing them in the process. Truly, her Devil Fruit power is terrifying.
8 Ryu Ryu no Mi, Model: Pachycephalosaurus
One of the recently introduced Devil Fruits, Ryu Ryu no Mi, Model: Pachycephalosaurus was eaten by Ulti, one of the members of the Tobiroppo of the Beasts Pirates. This Devil Fruit is incredibly powerful and as the name suggests, gives her the power to transform herself into a Pachycephalosaurus and its hybrid at will.
As a Pachycephalosaurus, Ulti gains tremendous physical strength that allowed her to compete with Luffy in Gear Second and Gear Third without any trouble at all and even forced him to consider using Gear Fourth against her. What's more, the headbutts that Ulti can deliver using this Devil Fruit are quite deadly and can easily shatter the bones in the body of a person.
7 Ryu Ryu no Mi, Model: Triceratops
Eaten by Sasaki, this Devil grants a high level of defensive power to its wielder, which is why Sasaki is the leader of the Armored Division of the Beasts Pirates.
When using its powers, Sasaki turns into a giant Triceratops whose power is superior to that of the Franky Shogun. It was able to knock the Shogun back with a powerful tackle, showing that it packs a tremendous level of strength. Sasaki prides himself in being the strongest creature to ever walk the earth.
6 Ryu Ryu no Mi, Model: Allosaurus
The Ryu Ryu no Mi, Model: Allosaurus was first introduced during the Sabaody Archipelago arc, however, it wasn't named until the Wano Country arc of One Piece. It was eaten by X Drake of the Drake Pirates at an unknown time. This Devil Fruit lets Drake transform himself into an Allosaurus, as seen first during the Sabaody Archipelago arc.
He can also use the hybrid form, which was first revealed during the Wano Country arc of One Piece. Drake's Allosaurus, being a carnivorous type of Zoan, grants him extra power when he is bloodthirsty. Furthermore, his Devil Fruit offers a great amount of resilience, as seen during his short clash with Sanji in the second act of Wano Country.
5 Ryu Ryu no Mi, Model: Spinosaurus
Another version of the Ryu Ryu no Mi, the Ancient Zoan Spinosaurus Devil Fruit was eaten by Page One, the youngest member of the Tobiroppo of the Beasts Pirates. This Zoan Devil Fruit lets him transform into a Spinosaurus and its hybrid at will. Since Spinosaurus are considered to be have been ferocious dinosaurs, Page One's Devil Fruit is certainly among the very best.
It grants him enough power to ravage a city with ease and an incredibly high level of durability. His jaw is the main source of strength in his animal form while his fists do most of the work in his hybrid form. Page One was strong enough to survive a battle against a Raid Suit-enhanced Sanji without taking much damage.
4 Neko Neko no Mi, Model: Saber-toothed Cat
This Devil Fruit was eaten by none other than Who's Who, one of the strongest members of the Tobiroppo. It allows him to become a cat of ancient times, known as the Saber-toothed Cat. Using the power of the Neko Neko no Mi, he becomes strong enough to fight even the likes of Jinbe.
Furthermore, this Devil Fruit grants him tremendous speed and physical power, as expected of an Ancient Zoan. Its power is similar to Rob Lucci's Neko Neko no Mi, Model: Leopard, however, far more dangerous, since it is a Saber-toothed Cat instead.
3 Zou Zou no Mi, Model: Mammoth
Considered to be one of the most powerful Zoans by the likes of Nekomamushi, the Zou Zou no Mi, Model: Mammoth is a rare Devil Fruit eaten by one of Kaido's strongest subordinates and a member of the All-Stars of the Beasts Pirates, Jack the Drought.
Jack has gained a bounty of a billion Berries and the powers offered by this Devil Fruit have certainly played quite a role in that. It allows him to transform into a Mammoth at will. Jack's Devil Fruit power offers incredible resilience, as seen during the Zou and the Wano arc of One Piece. He can take a tremendous amount of damage and fight for days, which makes him quite special indeed.
2 Ryu Ryu no Mi, Model: Brachiosaurus
The Ryu Ryu no Mi, Model: Brachiosaurus was eaten by another member of the Beasts Pirates All-Stars, Queen the Plague. This Devil Fruit is also among the strongest Ancient Zoans and it allows Queen to become a Brachiosaurus and its hybrid at will. Although this Devil Fruit isn't a carnivorous type, it certainly does make up for it with the sheer presence that it offers.
Damaging and defeating a Brachiosaurus is a task that requires a high level of strength, and what's more, Queen himself is quite powerful. His Brachio Bomber was powerful enough to halt even Big Mom in her tracks, which just goes to show how much strength he truly possesses.
1 Ryu Ryu no Mi, Model: Pteranodon
The strongest of all the Ancient Zoan Devil Fruits so far, the Ryu Ryu no Mi, Model: Pteranodon was eaten by King The Wildfire, the strongest member of the All-Stars and Kaido's strongest subordinate. This Devil Fruit lets him transform into a Pteranodon at will.
It is one of the very few Zoans that, along with great fighting prowess, offers the ability to fly as well. King's speed, strength, and durability are all incredibly high thanks to this Devil Fruit. He was able to push the Queen Mama Chanter off the waterfall at the entrance of Wano Country with ease and even managed to overwhelm Sanji in terms of physical strength, although the latter took no damage from his lethal attack.