Tyrannomon

Tyrannomon was an evil Digimon that is like a dark mirror of Greymon, even possessing a similar fireball-based attack. Many Tyrannomon serve Etemon. Tyrannomon can digivolve to DarkTyrannomon.

Comments

In the Digimon virtual pet game, Tyrannomon is a possible creature that Agumon can digivolve to- it is unknown if this is true in the series, but it would explain much. In DarkTyrannomon's description, it says that they were good until infected with a computer virus- perhaps Agumons become Tyrannomons when infected with such a virus.