Largo was a stray dog found by Pinoko, Wato, and Sharaku during a car accident, in "Thieving Dog". Contrary to the manga, she is saved after an earthquake caused Black Jack's house to collapse on top of her, and she becomes his pet dog. She is named Largo for her lethargic nature, but she compensates with her sharp senses in detecting danger.

(Source: Black Jack Wikipedia)