Rat
Rats spawn on a spacecraft when there is food left on the floor.
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 (F2) 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.