Mockingbird

• WWizards of the Coast2024-07-26
  Official Ruling
  If the copied creature has in its mana cost, X is 0.
• WWizards of the Coast2024-07-26
  Official Ruling
  You can choose not to have Mockingbird enter as a copy of another creature. If you do, it will just be a 1/1 Bird Bard creature with flying, an ability that didn’t do anything, and the opportunity for a good self-deprecating joke or two.
• WWizards of the Coast2024-07-26
  Official Ruling
  If the copied creature is copying something else, then Mockingbird enters as whatever that creature copied, with the noted exceptions.
• WWizards of the Coast2024-07-26
  Official Ruling
  Mockingbird copies exactly what was printed on the original creature, except it’s a Bird in addition to its other types and it has flying, and nothing else (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 and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
• WWizards of the Coast2024-07-26
  Official Ruling
  If the copied creature is a token, Mockingbird copies the original characteristics of that token as stated by the effect that created that token, with the noted exceptions.
• WWizards of the Coast2024-07-26
  Official Ruling
  Any “enters” abilities of the copied creature will trigger when Mockingbird enters. Any “as [this creature] enters” or “[this creature] enters with” abilities of the copied creature will also work.
• WWizards of the Coast2024-07-26
  Official Ruling
  Mockingbird’s second ability counts all mana spent to cast Mockingbird, not just mana spent on . If there are any additional costs to cast it, the mana spent to pay those costs counts as well.

Rulings via Scryfall