TT #38 · Parashas Vayeishev · Passuk 38:20
And Yehudah sent the kid of the goats by the hand of his friend the Adullamite, to receive the pledge from the woman’s hand; but he did not find her.
Torah Temimah points to Chullin 113b (classic on גדי).
Sefaria — Chullin 113b (classic on גדי)
Since the verse specifies “a kid of the goats,” learn from here that here it means kids of goats; but wherever “g’di” is stated without specification, even a cow or a ewe is included.
R’ Eliezer #1 — Ribbui / specification: when the Torah sometimes says “g’di of goats” and sometimes “g’di” alone, the bare term is inclusive.
“G’di” without qualification includes other species; the law of meat-and-milk is not limited to goat meat.