Now there remained nothing to do but invent a plausible explanation for their brother's disappearance to give to Jacob. First of all, however, they took an oath not to betray to his father or any human being what they had actually done with Joseph. He who violated the oath would be put to the sword by the rest. Then they took counsel together about what to say to Jacob. It was Issachar's advice to tear Joseph's coat of many colors, and dip it in the blood of a little kid of the goats, to make Jacob believe that his son had been torn by a wild beast. The reason he suggested a kid was because its blood looks like human blood. In expiation of this act of deception, it was ordained that a kid be used as an atonement sacrifice when the Tabernacle was dedicated.
The Brothers Swear a Deadly Oath to Hide Joseph's Fate
Legends of the Jews 1:45