See the Unwritten

If a card you put onto the battlefield this way has in its mana cost, X is considered to be 0.

You determine whether the ferocious ability applies before any creature cards are put onto the battlefield. If you put two creature cards onto the battlefield this way, those creatures enter the battlefield at the same time.

Because you’re not casting the creature cards, you can’t pay any additional or alternative costs. You can’t put the cards onto the battlefield face down.

Some ferocious abilities that appear on instants and sorceries use the word “instead.” These spells have an upgraded effect if you control a creature with power 4 or greater as they resolve. For these, you only get the upgraded effect, not both effects.

Ferocious abilities of instants and sorceries that don’t use the word “instead” will provide an additional effect if you control a creature with power 4 or greater as they resolve.