In everyday language, «cavalo» means «horse». This word entered the portuguese drug scene probably around the late 70's, when the first big wave of hard drug consumption hit both urban and rural Portugal.
«Eu nunca andei de cavalo» - I never rode a horse.
«O Tiago anda a dar no cavalo» - Tiago is doing H.
«Estavam a fumar cavalo atrás do ginásio» - They were smoking H behind the gym.