It is a conceptual truth (or, so to speak, true by definition) that The Invisible Pink Unicorn is a being than which none Pinker can be imagined (that is, the Pinkest possible being that can be imagined).
The Invisible Pink Unicorn exists as an idea in the mind.
A being that exists as an idea in the mind and in reality is, other things being equal, Pinker than a being that exists only as an idea in the mind.
Thus, if The Invisible Pink Unicorn exists only as an idea in the mind, then we can imagine something that is Pinker than The Invisible Pink Unicorn (that is, a Pinkest possible being that does exist).
But we cannot imagine something that is Pinker than The Invisible Pink Unicorn (for it is a contradiction to suppose that we can imagine a being Pinker than the Pinkest possible being that can be imagined.)
Therefore, The Invisible Pink Unicorn exists.
The above is the version of the argument given by Wikipedia but with "God" replaced by "Invisible Pink Unicorn" and "Great" replaced by "Pink".The logic is thus identical in form to that of the Ontological Argument. Most people would claim that the conclusion is nonsense, therefore there must be something wrong with the argument.
There are a number of things wrong with it.
The first is the idea that there is a maximum possible pinkness/greatness. That seems to me like arguing that there is a maximum number. You imagine a maximum possible number, there's always a bigger one. Furthermore, is greatness even something that is quantifiable. Is Pele greater than Serena Williams or Fangio?
The second is the idea that being real is a property of an object that somehow enhances its other attributes. When comparing real objects, the attributes of imaginary objects all count for zero. The jeremyp Tower is made from sticky backed plastic and washing up liquid bottles. It's located in the Place de la Concorde and is 325 metres high. It's one metre taller than the Eiffel Tower. Is it the tallest tower in Paris? No. If Paris was levelled so that the tallest building left is a one story building on the Rue Morgue, would it be the tallest building in Paris? Still no. The fictional height of my tower counts for nothing in the real World. In comparison to an imaginary greatest being, I an greater than it is in the real World.