Anguirus (Japanese: アンギラス, Hepburn: Angirasu) is a giant monster, or kaiju, which first appeared in the 1955 film Godzilla Raids Again, the second film in the Godzilla franchise produced and distributed by Toho. Anguirus is the first monster to be shown engaging in combat with Godzilla in a film. Since then, the character has appeared conversely as an enemy and an ally of Godzilla in numerous films produced by Toho, including Destroy All Monsters (1968), Godzilla vs. Gigan (1972), Godzilla vs. Megalon (1973), Godzilla vs. Mechagodzilla (1974), and Godzilla: Final Wars (2004). He has also appeared in other media, including comic books, video games and television series.

Anguirus is a quadrupedal giant or irradiated dinosaur that mostly resembles an ankylosaurus. His head resembles a cross between that of a ceratosaurus and a styracosaurus. He has several horns on the top of his head and a single horn above his nose. His face is long and drawn out, has rows of long, serrated teeth and has two tusks on his upper jaw. His carapace is studded with long, sharp spikes. Anguirus' tail is covered with spikes (he also had a tail club in Godzilla: Final Wars). His tail makes up most of his body length. His hind limbs are longer than his forelimbs, and he can stand up on them to his full height, though he generally walks upon all fours. Anguirus has not one but five brains, one in his head which controls all of his organs, and one above each one of his limbs; each brain most likely controls the limb it is above.^{[citation needed]} This gives Anguirus an advantage in battle, for he can react much quicker than most kaiju. Anguirus first appears in the Showa continuity as brown with yellowish spines, these colors are later changed in the Millennium series to light gray, with orange spikes and horns.

In his first appearance in Godzilla Raids Again, it is explained that Anguirus is capable of moving incredibly fast in spite of his bulk due to his brain extending into his chest and abdominal areas, allowing him to react more quickly. He is able to lunge at his opponents with massive leaps, as seen in Godzilla vs. Mechagodzilla, and is also capable of burrowing substantial distances. His long, burrowing journey started in Godzilla vs. Megalon and ended in Godzilla vs. Mechagodzilla, in which he resurfaces underneath Mechagodzilla and battles until his jaw is broken by the robot. Two of his attacks involve jumping backward to impale his opponents on his spiked carapace in Godzilla vs. Gigan and his vice-like bite; the most famous example of the latter being when he faced King Ghidorah in Destroy All Monsters and latched onto one of King Ghidorah's necks, his grip holding firm even after the space monster took flight. Anguirus also has the ability to heal and regenerate in a few minutes, as executives said he recovered almost immediately after Mechagodzilla broke his jaw. Anguirus has incredible durability, surviving being stomped on, being kicked with immense robotic force, being dropped from high altitudes, and surviving Godzilla's atomic ray.

In Godzilla: Final Wars, Anguirus had the ability to curl himself into a ball and propel himself forward with tremendous speed. The Final Wars version also possessed a spiked tail club like a real ankylosaurus, though it was not used in combat. He also had spikes on his legs.

Within Godzilla Raids Again, Anguirus' last roars, before being defeated by Godzilla, caused ultrasonic-like effect which produced cracks on Osaka Castle. Anguirus was also noted to be able to emit stunning poison from his claws and back spikes. Anguirus was also originally planned to breathe an atomic ray in Godzilla Raids Again, and this was recorded in the official novelization of the film.

During the existence of the character, there have been three official Anguirus suits. The first and second were constructed under the supervision of Eiji Tsuburaya. After the release of Godzilla Raids Again, Toho was looking to release the movie to Western audiences but was having trouble finding a distributor. AB-PT pictures, an American distribution company, was producing their own movie The Volcano Monsters shortly after the release of Godzilla Raids Again. AB-PT attempted to incorporate the monster footage of Toho's film into their own, but had little success. They struck up a deal between the two companies; the second Godzilla suit and the original Anguirus suit were shipped to Los Angeles to film some additional scenes. Due to financial problems with AB-PT studios, the company collapsed and its assets were absorbed into other production companies. The second Godzilla suit and the original Anguirus suit both disappeared and remain lost to this day. The second suit, created for color filming, was built in 1968. The design had no radical changes from the original aside from some modification on the thickness of the spines and arrangement of the teeth. For Godzilla: Final Wars, a third suit was built incorporating modern sculpting methods and animatronics. The final suit is darker in color and somewhat more streamlined and biologically realistic in design. Angurius' roars were created by using the sounds of a saxophone, a harmonica, an oboe, and a tuba.

Anguirus has been played by multiple actors throughout the Godzilla movie series:

According to the English subtitles of Godzilla Raids Again, Anguirus, or "Angilas" as he was referred to in the film, was an Angilosaurus, a fictional dinosaur that lived from 150 to 70 million years ago and is described as "one of the stronger dinosaurs that lived in the prehistoric era." Additionally, he is described in a textbook by Polish world animal specialist Plateli Hondon as "one of the few creatures that had a thorough hatred for war-like predators". An individual Angilosaurus was awakened by the same hydrogen bombs that awoke Godzilla and battled him until he was killed by the latter.