The Welsh Flag

"The Welsh flag has two equal horizontal stripes, white above green, and a large red dragon passant. The dragon standard was perhaps first seen in Britain in the shape of the "draco" a standard carried by the cohorts of the Roman legion. The Romans appear to have been inspired by the dragon standard carried by their Dacian and Parthian enemies and had adopted this device by the third century. Carl Lofmark (see below) argues that the dragon of the cohort was more familiar to the British than was the eagle standard of the legions. As Roman legions withdrew at the end of the fourth century and the British were left alone to face Saxon attacks, the dragon would have been a natural symbol for those who wished to preserve their Romanised way of life against the barbarian invader."

http://www.data-wales.co.uk/flag.htm