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If the copied creature had in its mana cost, X is considered to be 0.
If the copied creature was copying something else, then the token enters the battlefield as whatever that creature copied.
If the token is a copy of a creature whose power and toughness are defined by an ability (usually printed as */* or similar), the token doesn't copy the ability that defines its power and toughness. It remains a 1/1 creature.
The token copies exactly what was printed on the original creature and nothing else (unless that creature is copying something else; see below). It doesn't copy whether that creature was tapped or untapped, whether it had any counters on it or Auras or Equipment attached to it, or any non-copy effects that had changed its power, toughness, types, color, or so on.
The token copies the creature as it last existed on the battlefield before it died, not as it existed in the graveyard before it was exiled.
If something becomes a copy of the token, the copy is also a 1/1 and a Nightmare.
Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the creature will also work.
