The "Monster of Aramberri", also nicknamed in the scientific literature as the Aramberri pliosaur or the Aramberri specimen, is an informal name given to UANL-FCT-R2, a fossil skeleton of a very large pliosaur of which the first remains were discovered during the 1980s near the town of Aramberri, in Nuevo León, Mexico. The specimen was first interpreted as a dinosaur in 1988, before later inspections re-identified it as a marine reptile belonging to the family Pliosauridae. Initially, two concretions only containing the animal's fossils were discovered, with one of the two—containing the fossils of a rostrum and teeth—later noted as lost in the first in-depth re-identification of the specimen in 2003. During the early 2000s, a new excavation campaign unearthed several additional fossils of the animal, in which some of them were subsequently sent to Karlsruhe State Museum of Natural History, Germany, to be prepared, before returning them in 2012 to the Autonomous University of Nuevo León, where they are mainly stored. Another significant portion of the fossils are currently stored in the Desert Museum of Saltillo.
The "Monster of Aramberri" is one of the largest pliosaurs ever discovered, but estimates of its size have dropped considerably over the years. Initial estimates set a length of around 15 m (49 ft), with maximum proposals going up to 18 m (59 ft) or even 20 m (66 ft) according to some media. Its former interpretation as a juvenile as well as its incorrect attribution with Liopleurodon—then incorrectly portrayed in the 1999 BBC documentary series Walking with Dinosaurs—would probably have been the origin of the over-exaggeration of its size. However, the most recent accurate estimates put the size of the specimen at around 10 and 11 m (33 and 36 ft). Nevertheless, the animal would have an approximately 3 m (9.8 ft) long mandible and the large teeth since lost would have had possesed two sharp edges.
Based on various comparisons and descriptions, the "Monster of Aramberri" is most likely a representative of the Thalassophonea, a derived clade of pliosaurids characterized by a short neck and a large, elongated skull. The gastralia (abdominal ribs) of the Aramberri pliosaur possess traits that could be diagnostic for a distinct pliosaurid lineage that may soon be described. In the trunk, the Aramberri pliosaur preserves fossils of what appears to be an ichthyosaur, suggesting that this was its last prey consumed before its death. Two known cranial fragments of the animal also preserve bite marks that would have been made by another, more imposing pliosaur. The La Caja Formation, where the "Monster of Aramberri" was discovered, contains abundant marine fossils from a shallow environment dating from the Kimmeridgian stage of the Late Jurassic. It shared its habitat with a variety of other animals, including invertebrates, fish, thalattosuchians, ichthyosaurs, and other plesiosaurs.