Renegade Bull

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the spell.

For spells on the stack with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Renegade Bull's second ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack without resolving.

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are checked.

You cast the copy while Renegade Bull's last ability is resolving and still on the stack. You can't wait to cast it later in the turn.