He is a potential monster who takes down 3 Blue-Eyes White Dragons to defeat Kaiba.
He is a composite being between Red-Eyes B. Dragon and Meteor Dragon.
Yugi tells Kaiba that Red-Eyes B. Dragon does not bring the victory, only the potential, then Red-Eyes B. Dragon and Meteor Dragon fuse together via "Polymerization" who has the japanese name in manga "Fusion" by fans.
Meteor B. Dragon was seen in the Japanese animated movie Yu-Gi-Oh! Toei Movie where he was fused from Red-Eyes B. Dragon and Meteor Dragon into a composite being to take down Blue-Eyes White Dragon's 3-Body Connection, winning Yugi in the Duel.
In Yu-Gi-Oh! GX, Trueman used this card via "Polymerization" to fuse Meteor Dragon and King of The Swamp who acts as the substitute for Red-Eyes B. Dragon into Meteor B. Dragon, he was then defeated by Jaden Yuki.