black rubber type plug

help i have a 2002 harley fat boy carb type and found a black rubber type plug best way i can describe it is about 2 inches long solid it starts very thi for say 1/2 inch then thickens slightly and again increases its size,just cant fine a hole where it might go it was underneath the bike in my garage when i took it off the stand riser i keep her on so im sure its place is somewher near the rear of the bike god knows where ,,

would appreciate any ideas as i have no workshop manuel and in presenly in south america

regards ed,