Krypto was a creature of Krypton that resembled a dog of Earth. The creature belonged to the House of El and upon the destruction of Krypton, sacrificed itself into the Phantom Zone to save Kal-El. Kal, later Superman, later defeated Xa-Du to reunite himself with his trusted companion.
Sometime after Clark White had retaken the mantle of Superman, he found the Eradicator at the Fortress of Solitude attempting to remove the human DNA from Jon Samuel Kent. Krypto tried in vein to save Jon but was killed by the Eradicator before he attack him.