Seasonal Dragons were released along various dates and are one of the few dragons to possess different forms depending on the physical environment of Valkemare. The first Seasonal dragon to be released was the Summer variant, followed by Autumn, Winter and lastly Spring. As the name suggests, Seasonal dragon's variants follow the different seasons of the planet: a different colored adult and hatchling can be caught for each Spring, Summer, Autumn and Winter. ― (Forum FAQ)