Altobello could have been Pentangeli's succesor, or the Don of another family (probably not in NYC) that was allied with the Corleones. Or, he simply could have been a close friend of Vito, who honored him by making him Connie's godfather. Certainly he was connected to the Mafia in some direct way, although he wouldn't have had to have been an actual family head to have the honorary title Don.