An auto-regeneration unit was a Borg device. It was spherical in shape and could crawl around on its own autonomously even after its ship was destroyed.
An object suspected to be a functional auto-regeneration unit was amongst the technology salvaged from the debris of a Borg probe destroyed by the USS Voyager in 2375. Captain Kathryn Janeway initially mistook it for a transwarp coil. (VOY: "Dark Frontier")