No creature that can't be directly linked to Eastern myths will made it into a Zoan model. Marco's is the most Western creature we have yet it clearly represents the modern depictions of Fenghuang, the Eastern phoenix, not the Greek phoenix per se. Something like Cerberus doesn't fit, Oda even prefers to go for way less known beasts (Onyudo, Okuchi no Makami) than creating these Greek/Nordic/Whatever based models people like to theorize about. Even when given the opportunity to freely design an in-verse god (Nika) he gave it explicit Japanese traits.
In fact, whenever he wants to include a mythical beast that isn't Eastern he follows different routs: Jabra, the Boa sisters, Pekoms, Chaka, Pell, Pierre, Thriller Bark's Cerberus, the monsters from Impel Down...; all of them represent non-Eastern myths yet none has a mythical Zoan.