Polimorfa di Jwar

Any "enters" abilities of the copied creature will trigger when Jwari Shapeshifter enters.

If the chosen creature is a token, your Jwari Shapeshifter copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Your Jwari Shapeshifter is not a token.

If the chosen creature is copying something else (for example, if the chosen creature is another Jwari Shapeshifter), then your Jwari Shapeshifter enters as whatever the chosen creature copied.

Jwari Shapeshifter copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

You can choose not to copy anything. In that case, Jwari Shapeshifter enters as a 0/0 creature, and is probably put into the graveyard immediately.