I definitely don't think that an Imaginary Friend would be considered a "monster" by the game. And how do you create it?
You can have more than one built/bought SimBot if you have the necessary ingredients/enough LTHP. The monsters don't have to be different, so the easiest way got to be to rack up 80K happiness points and buy two SimBots and wait for the Master Invention opp to finish the LTW.