The "Monster of Aramberri" is an informal name given to a fossil skeleton of a very large pliosaur since numbered as UANL-FCT-R2, of which the first remains were discovered in 1985 near the town of Aramberri, Mexico. In scientific literature, some authors also refer to it as the Aramberri pliosaur or the Aramberri specimen. Initially interpreted as a dinosaur in 1988, it was formally reidentified as a marine reptile of the family Pliosauridae in a short 2001 publication. 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 study conducted on the specimen in 2003. During the 2000s, a new excavation campaign unearthed several additional fossils of the animal. Subsequently, most of these fossils were 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. Most recent accurate size estimates put fixes 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 possessed two sharp edges.

Since 2013, the "Monster of Aramberri" is viewed as 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.

During an excursion conducted in the fall of 1985, in the Mexican state of Nuevo León, a geology student of the Autonomous University of Nuevo León reported the discovery of a large concretion of fossils in the Sierra Madre Oriental, at around 1 km (0.62 mi) northwest of the municipality of Aramberri. During the exhumation, a second, equally large concretion was discovered by paleontologists. The inaccessibility of the place and the fossils collected at the time weighing around 200 kg (440 lb) in total, prevented researchers from moving it immediately. The following year, the discovered material was finally moved over 1.24 miles (2.00 km) via fairly complex technical processes before reaching a road facilitating transport. Subsequently, the two large concretions were sent to the Faculty of Earth Sciences of the Autonomous University of Nuevo León, where the fossils concerned are all cataloged under the specimen number UANL-FCT-R2. The preparation of the first fossil concretion lasted two months, and when this was finalized, it was fixed vertically on a poured concrete base. The first concretion contained numerous postcranial elements which include, among other elements, seven pectoral vertebrae (transitional vertebrae between the cervical and thoracic regions), fragments of coracoids and a left scapula, ribs as well as gastralia (abdominal ribs). The second concretion contained a rostrum with teeth. These fossils were first mentioned in the scientific literature in 1988 by German geologist Walter Hähnel, one of the researchers who participated in their exhumations. In his paper, he indicated that the discovery had been made stratigraphically within the La Casita Formation, a Late Jurassic geological formation more precisely dated to the late Kimmeridgian. Despite the fact that the specimen was discovered in marine sediments, Hähnel identified it as a carnivorous dinosaur with a length ranging between 10 and 15 m (33 and 49 ft).

In September 2001, German paleontologist Eberhard Frey [de] and colleagues announced via a publication that they had re-identified the specimen as actually coming from an imposing marine reptile of the family Pliosauridae on the basis of its vertebral morphology, while also pointing out that the fossils should be described in more detail later. Still in the announcement, the authors give this specimen the informal nickname of the "Monster of Aramberri". From 2002, this specimen received international attention from the media, and the nickname given has since been used by both journalists and scientists. The announced study describing the fossils in more detail was finally published in 2003 and was led by French paleontologist Marie-Céline Buchy and her colleagues. In the 2001 publication, the authors note that the second concretion was not relocated to the university, but it is since the 2003 study where it is noted as lost. Based on the stratigraphic position of various ammonite specimens at the site, the authors of the latter study also revealed that the Aramberri pliosaur was in fact discovered in the La Caja Formation, which dates to the "middle" Kimmeridgian.

From 2001 to 2007, new expeditions carried out in the type locality by Mexican, French and German paleontologists helped by residents of the city, made it possible to exhume the caudal part of the specimen, and two thirds of the skeleton. Cranial fragments have also been discovered although most of them are unidentifiable. Field data even suggested that the fossil specimen would have been technically complete before the erosion of the skull and its exhumation for phosphorite. Ultimately, it seems that 70% of the animal's skeleton is known. In 2003, in order to help paleontologists, the newly elected mayor of Nuevo León sent a helicopter to transport a fossil block weighing a total of 450 kg (990 lb). During his campaign the following year, the mayor approved and even completed the construction of a road leading to the excavation site. Once exhumed, most of the fossils were transferred to the Natural History Museum in Karlsruhe, Germany, for preparation. The Karlsruhe Museum could not accommodate more fossils due to the size of the animal, so the remaining material was transferred to the Desert Museum in Saltillo, Coahuila. When the material sent in Karlsruhe were prepared, they were returned to the Faculty of Earth Sciences of the Autonomous University of Nuevo León in 2012, where they have since been stored. Some fossils were temporarily exhibited in 2007 at the Mexican History Museum in Mexico City. The newly discovered and prepared fossils consisted of nine cervical vertebrae preserved on three blocks, additional dorsal vertebrae, ribs and gastralia, a femoral joint head as well as an almost complete pelvic girdle. The cranial elements identified included a pterygoid, a jugal and part of a maxilla preserving a dental alveolus. The new fossils are still being prepared and should receive much more detailed descriptions in future work. In 2012, Mexican paleontologist Javier Aguilar Pérez expressed that the specimen should receive a skeletal mount formed from the fossils once their preparations are completed. Furthermore, he also expressed that new cranial fragments should be found in the field where the specimen was discovered, but no expedition has apparently been sent since.

The Aramberri specimen is one of the most complete large pliosaurs ever discovered. Although the entire skeleton of the animal has not received detailed descriptions at present, authors have nevertheless described certain parts of the animal. Plesiosaurs are usually categorized as belonging to the small-headed, long-necked "plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype, the "Monster of Aramberri" belonging to the latter category. Like all plesiosaurs, it had a short tail, a massive trunk and two pairs of large flippers.

The "Monster of Aramberri", as its nickname suggests, is also one of the largest pliosaurs identified to date, but its size estimates have declined considerably over years. Initial estimates putted the size at around 15 m (49 ft) long at least, although maximum estimates of up to 18 m (59 ft) with a body mass of 50 t (49 long tons; 55 short tons) was also proposed. In 2008, British paleontologist Adam S. Smith and his Irish colleague Gareth J. Dyke, citing the 2003 paper, give a maximum length of 17 m (56 ft). In his 2009 thesis, Australian paleontologist Colin McHenry criticized this interpretation, which he finds very exaggerated. Comparing the Aramberri specimen with the fossil material attributed to Kronosaurus, he reduced its size to between 11.7 and 12.2 m (38 and 40 ft) for a body mass of between 14.9 and 17.8 t (14.7 and 17.5 long tons; 16.4 and 19.6 short tons). In 2014, Frey and Wolfgang Stinnesbeck increased the length of the specimen slightly to between 12 and 14 m (39 and 46 ft). In 2021, German paleontologists Frederik Spindler and Martin Mattes further reduced the size of this specimen to between 10 and 11 m (33 and 36 ft) long. In 2024, Chinese paleontologist Ruizhe Jackevan Zhao does not give a precise estimate of the measurements of the specimen. He suggests instead, based on vertebral dimensions, that it would have been similar in size to Pliosaurus funkei, which according to his model was approximately 9.8 m (32 ft) long with a body mass of 12 t (12 long tons; 13 short tons). The same year, Australian biologist Joel H. Gayford and his colleagues estimated that the animal would have reached approximately 10.7 m (35 ft) in length.