The Dragon of the Beginning (始まりの竜 Hajimari no Ryū) is a term that refers to both a title and a special Tribe found in all colors.
According to the story, the "Dragon of the Beginning" title itself specifically refers to the Dingir "Enki", the originator of Energeia and the Dragons. In the Z/X -Zillions of enemy X- TCG, the "Dragon of the Beginning" term is also used as a collective name of the majority of "Dragon" Tribe.
Z/X Encyclopedia Entry
In the Z/X -Zillions of enemy X- TCG, the Dragons are first introduced in Set 3 and received their first support in Extra Pack 7. Before their introduction, their existence are subtly hinted through the Ignition Icon (which resemble the shape of a dragon's head) and the Dragon Miko herself.
According to official source, the Dragons in story are separated into several groups, with the most notable being the Dragon Mikos' Partner Dragons and the titular "Five Dragon Emperors" (五帝竜 Goteiryū) from Set 3. Their relation with each other and the Dingir Enki, if any, is still largely unknown at this time.
As shown with the "Five Dragon Emperors", each individual Dragons has their own gender and also able to take on Humanoid Form (人間形態 Hyūmanoido Fōmu) if they will.
In the Z/X -Zillions of enemy X- TCG, the Dragon Tribe is separated into several Tribes based on their color instead, all contains the word "Dragon" (ドラゴン Doragon) in the Tribe name. They are collectively supported through the "...with [Dragon] in (its/their) Tribe" clause in the card text.
Glorious Dragon (グロリアスドラゴン Guroriasu Doragon) are the Dragon Tribe of Red World. They resemble generic western dragon with bipedal, slightly humanoid bodies, reddish coloration, and wore various sorts of armor and weaponry not unlike medieval knights. Their abilities mostly deals with direct damage to opponent's Z/X and the usage of Remove Zone.
For the list of all Glorious Dragon, see Category:Glorious Dragon.
Gear Dragon (ギアドラゴン Gia Doragon) are the Dragon Tribe of Blue World. They resemble eastern dragon with serpentine and fully mechanical bodies. Their abilities deals with all sorts of Blue portion of effects, mainly card draws and returning opponent's cards.
For the list of all Gear Dragon, see Category:Gear Dragon.
Angelic Dragon (エンジェリックドラゴン Enjerikku Doragon) are the Dragon Tribe of White World. They resemble western dragon with mostly bipedal bodies, light coloration and/or armor across their bodies, and true to their name—they possessed one or more pairs of feathery wings. Their abilities are mostly centered around Event cards, with other White portion of effects and various protective effects mixed in.
For the list of all Angelic Dragon, see Category:Angelic Dragon.
Cruel Dragon (クルーエルドラゴン Kurūeru Doragon) are the Dragon Tribe of Black World. They has mostly bipedal bodies with dark coloration and numerous spiky scales or blade-like appendages protruding from the parts of their bodies. Their abilities mainly deals with direct destruction and Trash management.
For the list of all Cruel Dragon, see Category:Cruel Dragon.
Vine Dragon (ヴァインドラゴン Vain Doragon) are the Dragon Tribe of Green World. They resemble a sort of cross between western and eastern dragon, with bipedal and bulky elongated bodies, large visible scales, and various vegetation growing out of their bodies. Like the majority of Green cards, their abilities mainly deals with the Resource.
For the list of all Vine Dragon, see Category:Vine Dragon.
True Dragon (トゥルードラゴン Turū Doragon) are the form of dragons "before their color were determined". They has more varied appearances with silver and gray as their dominant color. Their cards are exclusively colorless and also wield various range of abilities in contrast to the other Dragon Tribe.
For the list of all True Dragon, see Category:True Dragon.