Imannuel Kant : Ontological argument of existence

In Immanuel Kant's hypothesis of the ontological argument for existence, he states that 'existence' is not a synthetic judgement of a concept. The concept of an existing horse and a non existing horse are identical. If a concept exists, to say it is 'existent' is simply a tautology, and adds no differentiation to the object or the concept being defined. The distinction between horses and unicorns is not that unicorns are 'non existent' but rather we have spatio-temporal awareness of horses, and through this we can relate the concept 'horseness' to a particular object in a particular space and time. By the same premise, the concept of the existent floating aborigines with a silver moose head is no different from the concept of the non existent floating aborigines with a silver moose head.