The coats of arms of both England and Scotland date back to the very earliest days of heraldry, the later 12th century. King William I of Scotland chose to have a red lion on a yellow ground in about 1180. The double border of red lines was added later. This lion is
shown in a position known as ‘rampant’, chosen as this allows a single lion to fit easily on to a triangular shield.
The kings of England had been using two yellow lions as a symbol since the 11th century. This was before formal heraldry evolved, so the lions were shown in all sorts of poses. It was Richard I who produced the modern arms in the 1190s. His mother was Eleanor, Duchess of Aquitaine, who used arms of a single yellow lion on a red field. The lion was shown looking at the viewer while walking to the left – a pose known as passant-guardant. The two English lions were added to the single one of Aquitaine to produce the arms of England we know today.
As to why the rulers of England, Scotland and Aquitaine all chose lions, it is simply that lions are bold, strong and fierce creatures – all attributes thought to be important in a medieval ruler.