Rulings

• 2018-01-19
  If Protean Raider somehow enters the battlefield at the same time as another creature, Protean Raider can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
• 2018-01-19
  Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn't have to still be on the battlefield. Similarly, the player or planeswalker it attacked doesn't have to still be in the game or on the battlefield, respectively.
• 2018-01-19
  Protean Raider copies exactly what was printed on the original creature (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.
• 2018-01-19
  If the chosen creature has in its mana cost, X is considered to be 0.
• 2018-01-19
  If the chosen creature is copying something else (for example, if the chosen creature is another Protean Raider), then Protean Raider enters the battlefield as whatever the chosen creature copied.
• 2018-01-19
  If the chosen creature is a token, Protean Raider copies the original characteristics of that token as stated by the effect that created the token. Protean Raider is not a token in this case.
• 2018-01-19
  Any enters-the-battlefield abilities of the copied creature will trigger when Protean Raider enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.
• 2018-01-19
  Raid abilities care only that you attacked with a creature. It doesn't matter how many creatures you attacked with, or which opponent or planeswalker controlled by an opponent those creatures attacked.