The brothers had to produce evidence. Targum Pseudo-Jonathan on (Genesis 37:31) explains their choice of weapon-of-deception with clinical precision: they killed a kid of the goats, because his blood is like the blood of a man.
Goat blood, in color and viscosity, most closely resembles human blood. The brothers were not improvising. They had thought it through. Which animal's blood would look, to their father's horrified eye, like the blood of his missing son?
And of all the animals they could have chosen, they chose a goat.
The sages never forgot this. Jacob himself, decades earlier, had deceived his own father Isaac by dressing in the skins of goats to impersonate Esau (Genesis 27:16). A young Jacob had used a goat to steal a blessing from his father. Now ten sons of Jacob use a goat to steal a son from their father. The measure is being returned. The deception travels through the generations.
This is the principle the sages call middah k'neged middah — measure for measure. Not as punishment, exactly, but as revelation. What we plant in one generation often ripens in the next. The goat that helped Jacob lie to Isaac returns, transformed, to help Jacob's sons lie to Jacob.
The Targumist states the fact without commentary: his blood is like the blood of a man. But the echo is inescapable. In the house of Israel, the old deceptions do not die. They wait in the fields, wearing the coat of a goat, until the next generation needs them.