Grub, Storied Matriarch

If the copied creature has {X} in its mana cost, X is 0.

If you are instructed to put a card that isn't a double-faced card onto the battlefield transformed, it will not enter at all.

In some rare cases, this permanent might transform while its triggered ability that allows you to pay mana to transform it is still on the stack. (For example, another effect might cause that ability to trigger an additional time.) In such a case, even if you pay mana as the triggered ability resolves, this permanent won't transform because it already transformed while the triggered ability was on the stack.