The Draco Cargo Capsule is a medium-sized premium part which is part of the NooleusX reusable rocket. This multi-stage rocket is comprised of several different stages, parts and payloads. It is an add-on (similar to the Transponder Module and Quick Launches) and thus only available to users via purchase. It can not be unlocked via gameplay, unlike most other rockets.
Though the Draco Cargo Capsule is part of the NooleusX rocket system, it may be mixed and matched with non-premium rocket components for launch.