plesiosaur

Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Generally depicted having an ovoid body, four flippers, long neck, and a comparatively small head. It's notable that not all real-life plesiosaurs have long necks, so this tag applies to any creature with the basic body plan of a plesiosaur.