If it's an automatic, this is likely the transmission breather tube, it should reach up and hook to a clip right around the battery tray, do not plug it.
when it is in a rubber casing or its a cell phone
The Haynes Repair Manual says to use 5W 30 engine oil if you have over a hundred thousand miles. for an 88 shadow there is a rubber plug on the drivers side of the transmission, about halfway up the casing. you know if you need to add more fluid if you can remove the rubber plug without anything leaking out. If it seems dry, stick your finger into the hole and try to feel the fluid level. to add more fluid to it you have to use a funnel with a rubber hose attached to it and gravity feed the oil into the transmission.
under the rubber of the front left tire under the rubber of the front left tire
The fabric and steel wires inside the rubber casing.
that is the transmission return cooler line
Back then they used pigs bladder with a leather casing. Now they use a rubber balloon as the inside.
the rubber casing on the outside of the cable is what protects it from damage.
Make sure you use rubber fuel line
rubber is used for things such as rubbers, tyres, sometimes wire casing, sometimes walets, handbags, intertubes for trucks i read somewhere that leftover rubber from tyres are used for ground mulch i hope this helped you a bit ! :D
The oil drain plug runs out of your oil tank and down through the casing that houses your transmission, it is located in the middle of the casing, in between the frame rails. It has a black plastic plug in the goodyear rubber hose. You will need a flat tipped screwdriver to loosen the clamp that holds the plug in. It is pretty much in line with your kick stand if you have your bike leaning on it.
it never comes out
If you are talking about a truck with a standard transmission, it could be coming in from the bottom of the shifter, the rubber protector between the trans and the cab might be loose. You may also have a vent opened.