An "attacking creature" is one that has been declared as an attacker this combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left combat. There's no such thing as an attacking creature outside of the combat phase.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.