Cheapest priced copy (Rivals of Ixalan): $0.49 at Card Kingdom, near mint, their price list as of .
Protean Raider 
Creature — Shapeshifter Pirate · 2/2 · Rare · Rivals of Ixalan · #167
Raid — If you attacked this turn, you may have this creature enter as a copy of any creature on the battlefield.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Rivals of Ixalan Promos | 167s | Rare | — | $0.79 |
| Rivals of Ixalan | 167 | Rare | $0.49 | $0.99 |
8 rulings from Wizards of the Coast
- 19 January 2018If 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.
- 19 January 2018If the chosen creature has {X} in its mana cost, X is considered to be 0.
- 19 January 2018If 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.
- 19 January 2018Any 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.
- 19 January 2018If 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.
- 19 January 2018Protean 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.
- 8 November 2024Raid abilities care only that you attacked with a creature. It doesn't matter how many creatures you attacked with or which player, planeswalker, or battle those creatures attacked.
- 8 November 2024Raid 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, planeswalker, or battle it attacked doesn't have to still be in the game or on the battlefield.