The Celts believed dragons to hold great wisdom and knowledge. Their very presence could affect the flow of magic, as they travelled through the land they left behind the lines of energy (ley lines). Any place the lines crossed was a place of increased power.
(e.g Stonehenge, Newgrange). They were much revered by the celts and were adopted as a symbol of power for their chieftain/king. Hence the name Pendragon (the Celtic name for King orWar Chief).
The green dragon is the is the Celtic Animal sign for people born from January 21st -February 17th and is believed to breathe fire that as the power to purify and give life.