Rat
Rats are a kind of animal that spawn on a spacecraft when there is food left on the floor. They are generally small and will appear out of nowhere to try and eat whatever food they can find. Rats are normally not hostile or pose much of a threat, but on occasion they can have special attacks with poison, disease, or worse.
Once rats spawn, the spacecraft will be marked as having a nest. While this nest exist the rats will continue to spawn.
Each spacecraft has its own rat DNA, so each spacecraft's rats are unique.
Exterminator
Rats are removed from a spacecraft by requesting an exterminator on the Communication (F3) window's Trade channel, using the "Service Spacecraft" option.
The exterminator will remove all rats and their nest instantly. All food commodities laying on the floor are also removed.