You may choose only a creature that’s already on the battlefield. : If Mirror Image somehow enters the battlefield at the same time as another creature, it can’t become a copy of that creature. Any “as enters the battlefield†or “ enters the battlefield with†abilities of the chosen creature will also work.
: Any enters-the-battlefield abilities of the copied creature will trigger when Mirror Image enters the battlefield. Mirror Image doesn’t become a token in this case. Interestingly, there are two cards that sit outside of these colors: Brutal Cathar is. As of Innistrad: Crimson vow, there are four black, 12 red, 13 green, four Gruul (red/green), one Boros (red/white), and one Simic (blue/green). : If the chosen creature is a token, Mirror Image copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Daybound and Nightbound, the transforming cards, are primarily found in the Jund colors of black, green, and red. : If the chosen creature is copying something else (for example, if the chosen creature is another Mirror Image), then Mirror Image enters the battlefield as whatever the chosen creature copied. : If the copied creature has in its mana cost, X is considered to be 0. It doesn’t copy whether that creature is tapped or untapped, whether it as any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on. : Mirror Image copies exactly what was printed on the original creature (unless that creature is copying something else or is a token see below).