Egyptian art sometimes demonstrates the king’s power by showing him as a powerful animal, such as a lion or sphinx. This was a reminder that he was a god, who could appear in many forms.
The king was also represented as a strong bull, which gave rise to the “Festival of the Tail” – the Heb Sed. Part of the king’s costume was actually a bull’s tail.
During the Sed festival, the king had to perform physical activities, such as a ceremonial run, to renew his strength and show that he was still fit. Sed festivals were supposed to happen when a king had reigned for thirty years. But kings often held them more often, especially if their strength was failing or after some kind of disaster.
Source: The Usbourne Encyclopedia of Ancient Egypt