What animal did the father have killed to celebrate the prodigal son's return?

Answer: The fattened calf

"Bring the fattened calf and kill it. Let us feast and celebrate. For this son of mine was dead and is alive again!"

“Bring the fattened calf and kill it. Let us feast and celebrate. For this son of mine was dead and is alive again! He was lost and is found!’ So they began to celebrate.”— Luke 15:23-24 (BSB)