Here it is the most voluminous aircraft transporter in the world, the Airbus A300-600ST Beluga while loading in aircraft fuselage inside. The ST, stands for super transporter, this aircraft able to carry over-sized air cargo across continents. Usually used to transport aircraft parts, such as fuselage, wings, vertical stabilizer and so on. In a simple thoughts aircraft inside aircraft.
The Beluga built on Airbus A300 series platform with some major modifications on upper fuselage, engines as well as cargo-ing mechanism. The super transporter only able to load cargo from front-end, another cargo aircraft loads stuff from aft section or rear-end.
Recently, Airbus to replace the Beluga into Beluga XL in 2020. The XL is a modified Airbus A330 series but has better technology and more economical operating cost. The airlifter is even more important nowadays, since aircraft manufacturer spread away its parts around the world to its vendor or supplier. This could save money to invest and time saving as well. Aircraft manufacturer only does the assembly on their facility.