If my opponent played Stalking Wolf and they cannot pay the additional cost to sacrifice Poro, Dog or Cat, do they get to untap the Runes back?

According to the rules of Riftbound, if an opponent attempts to play Stalking Wolf but cannot pay the mandatory additional cost (killing a Bird, Cat, Dog, or Poro you control), they cannot play the card.

Regarding the runes:

1. Costs are paid during the process of playing the card: According to Rule 357.1 and Rule 404.1, costs are paid as part of the sequence of playing a card. If a player cannot pay a mandatory additional cost, they are unable to complete the process of playing that card.
2. State Reversal: If a player communicates an intent to play a card and begins the process, but realizes during the payment step that they cannot legally fulfill the requirements (or chooses to stop because they cannot pay), the game state should be rewound to the point before they began the attempt.
3. Runes are not "tapped": In Riftbound, runes are "exhausted." If the player exhausted runes specifically to pay for the Stalking Wolf (which they cannot finish playing), they are entitled to ready those runes again, provided no other game actions have occurred in the meantime (Rule 509.4.d.1).

Summary: Your opponent does not "untap" (ready) the runes because they were never successfully spent; rather, because the card cannot be played, the attempt to play it is invalid. They should ready the runes that were exhausted for the attempt, returning the game state to how it was before the illegal action was initiated.