The Singtwee is a Maxis-made creature present in the final game. It is similar to the creatures Singwon and Singtoo. It is the smallest of the three, unlike the other two it is green and has the default hands and feet. Like the others it has a row of bony protrusions on its back, providing it with charm. It can sing but not very well.
The "twee" ending may be a simplification of the number three.