In what did Joseph's brothers dip his robe to deceive their father?

Answer: A young goat's blood

They slaughtered a young goat (KJV: a kid of the goats), dipped Joseph's robe in its blood and sent it to their father.

“Then they took Joseph’s robe, slaughtered a young goat, and dipped the robe in its blood. They sent the robe of many colors to their father and said, “We found this. Examine it to see whether it is your son’s robe or not.””— Genesis 37:31-32 (BSB)