Soon after the birth of his son, Jor-El genetically altered a Kryptonian hound to increase its intelligence and loyalty so that it would serve as a companion and guardian for his son. Later, as the planet Krypton neared its destruction, Jor-El tried to save his family by escaping into the Phantom Zone.
The prisoners trapped within the Zone saw this as an opportunity to escape but Krypto fiercely defended his family and threw himself at the Phantom Zone portal, driving the prisoners back. Unfortunately, his attack damaged the portal and he was trapped within the Zone.
Thanks to the unusual qualities of the Phantom Zone, Krypto was able to follow Kal-El to Earth and kept watch over him. When the prisoners of the Zone once again tried to escape, Krypto came to Kal-El’s aid and the two drove back the invasion. Now, finally reunited with his beloved master, Krypto fights alongside Superman as he protects the Earth.