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the answer to your question is fairly simple. The version of the Gauth that you have linked to is a conversion of a monster from a previous edition of D&D, that was done in 2001. the MM 3.0 did not have a gauth in it, so we at the Creature Catalog did a conversion of our own. the MM 3.5 was released in 2004, and Wizards of the Coast decided to add their own version of the Gauth to that book.