Vibes, I think that you are referring to a trailing cone. This is a conical ‘bucket’ that enables measurement of the free air static pressure at some distance from the airframe which could induce false static readings. It is smaller than the in-flight refuelling hose and drogue system.
A cone is often seen on Boeing test aircraft, where on the ground the cone and pipe/cable hang down from the top of the fin; other manufacturers use an airborne deployable system that extends from a drum in the fuselage.