Green’s Dictionary of Slang

(drugs) heroin.

1969		R.R. Lingeman Drugs from A to Z (1970).
1981		D.E. Miller Bk of Jargon 337: caballo: Spanish for horse, i.e., heroin.
2001		ONDCP Street Terms 5: Caballo (Spanish) — Heroin. [Ibid.] 13: Kabayo — Heroin.