There are many races and civilizations represented in the population aboard Nibiru.  Some are more predominant than others. Some are hominoid, some reptilian, some insectillian, some android, etc.  Some are even like hairy apes.  There are literally hundreds of races and hybrid varieties represented in the population of Nibiru.  Once again I reference Deep Space Nine and also Star Trek. 

Each group of beings have living areas that simulate their natural environment.  For example the dolphin representatives live in an aquatic environment while the reptilians live an a hot and humid environment.  The humans live in an environment much like ours on earth.  The octopus like beings live in tanks made especially for them and so on.  Since Nibiru is almost 4 times the size of earth, there is plenty of room to move.  Some groups have developed into entire civilizations as a result and don’t return to their home worlds as often as the others.