Rats are various medium-sized, long-tailed rodents. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus. Many members of other rodent genera and families are also referred to as rats, and share many characteristics with true rats.
The crew of the U.S. starship Admiral Peary had rats for experimental purposes when the ship arrived at Home in 2031. Some of these rats escaped and thrived in this new environment, despite being very different from all other life forms on the planet. The efforts of the Race authorities to exterminate them proved futile, and they soon showed clear signs of becoming a permanent part of Home's ecology.
While the humans had not deliberately released the rats into Home's environment, some of them were not completely sorry it happened; they remembered the great damage to Earth's ecology due to creatures from Home being set free there, and the indifference shown by members of the Conquest Fleet and Colonization Fleet to humans' complaints about this.