Silbido de Yuna

If the revealed card has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

You don't choose a target for Yuna's Whistle at the time you cast it. Rather, a second "reflexive" ability triggers when you reveal a creature card this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.