EXTRA · Parashas Vayeishev · Passuk 37:31 · not a Torah Temimah citation on this passuk
Then they took Joseph’s tunic, slaughtered a kid, and dipped the tunic in the blood.
Classical source (EXTRA side-ring): Bereshis Rabbah 84:19.
Sefaria — Bereshis Rabbah 84:19
English: “And they slaughtered a goat” — why a goat? Because its blood resembles human blood.
Derash on the verse (Bereshis Rabbah) — not a formal named middah of the 13. Species choice is explained by visual likeness of the blood.
They chose a goat so the blood on the coat would look like human blood and complete the lie.