In the Elseworlds story JLA: The Nail Krypto is an early product of an experiment in splicing Kryptonian DNA to Earth creatures.
Krypto, also known as Krypto the Superdog, is Superman's pet dog in the various Superman comic books published by DC Comics.
They are primarily the foes of Ace the Bat-Hound, the pet of Batman, and were first introduced in the episode "The Dark Hound Strikes!" They attempt to commit crimes for their master, requiring Ace and Krypto (and Streaky the Supercat, on occasion) to stop them.
The strip was mentioned in MTV Geek and discussed in the Cipher Mysteries blog of cryptology expert Nick Pelling as well as Klausis Krypto Kolumne of cryptology expert Klaus Schmeh.
In Superman (vol. 2) #170, Jeph Loeb wrote a very controversial story where, in defending Lois Lane from certain death, Krypto almost killed Mongul by ripping his throat out.