What component can this be?
It's a GDT

What you're looking at is almost certainly a Gas Discharge Tube, or GDT for short.  They are basically a spark gap inside a sealed chamber filled with a specific inert gas mixture -- this allows them to provide more precise protection than an air spark gap.  They're used for the same things as spark gaps, though -- mostly, overvoltage protection as when they break over and start conducting, they will "crowbar" the overvoltage.