Agumon X is described as looking like a normal Agumon the only difference is that he has blue lines running across his body, such of those of his Digivolved forms.
He starts of the story getting chased by a herd of Mammothmon, they then enter the cave of KoKabuterimon X, a Boarmon enters, and the caves shakes, and they are teleported into the Human World. They meet KoKabuterimon X's Human Friend, Dave. His friends enter the room, and they introduce themselves. The Digimon then cause a reaction with the DigiDestined's Phones, and they gain a new app.
The name "Agumon" refers only to the Rookie form of this Digimon. Throughout the series, Agumon gains the ability to digivolve into a number of more powerful forms, each with a different name and special attack. However, the Rookie level is his preferred one and the one he spends most of his time in.
Greymon X is the evolved form of Agumon X, and is described as a massive dinosaur, with blue lines running across his body, and a massive head cover with three equally massive horns.
Greymon X's Evolved Form, and Agumon's Ultimate Form. He is a Greymon with metallic Upgrades to his body. A massive Metallic Arm, called the Trident Arm V99.9. He now has a metallic body frame, and gained two rocket blasters.