Malleable Impostor

Any enters-the-battlefield abilities of the copied permanent will trigger when Malleable Impostor enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the chosen permanent will also work.

Except for the listed exceptions, Malleable Impostor copies exactly what was printed on the original permanent and nothing more (unless that permanent is itself copying something or is a token; see below). It doesn't copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras attached to it, or any noncopy effects that have changed its power, toughness, types, color, and so on.

You can choose not to copy anything. In that case, Malleable Impostor simply enters the battlefield as a 0/0 creature and is probably put into your graveyard immediately, unless something else is increasing its toughness to keep it alive.