Cadborosaurus, nicknamed Caddy by journalist Archie Wills, is a sea serpent in regional folklore of the Pacific Coast of North America. Its name is derived from Cadboro Bay in Victoria, British Columbia, and the Greek word saûros (σαῦρος) meaning lizard or reptile.

Cadborosaurus willsi is said by witnesses to resemble a serpent with vertical coils or humps in tandem behind a horse-like head and long neck, with a pair of small elevating front flippers, and either a pair of hind flippers or a pair of large webbed hind flippers fused to form a large fan-like tail region that provides forward propulsion.
Paul LeBlond, director of Earth and Ocean Sciences at UBC, and Edward Blousfield, retired chief zoologist of the Canadian Museum of Nature, note the diverse array of elongated animals that have been put forward as an explanation for Caddy, including Conger eels, humpback whales, elephant seals, ribbon or oarfish, basking sharks, and sea lions. LeBlond and Blousfield state that no known creature matches the characteristics found in over 200 sightings collected over a century, noting that Caddy is described as having flippers both anteriorly and posteriorly. Darren Naish contends that LeBlond and Blousfield are engaging in "bad science" and have incorrectly assumed that different, conflicting eyewitness reports are all descriptions of one species when various accounts "are most parsimoniously interpreted as descriptions of many things".
In 1943, two police officers, Inspector Robert Owens, and Staff Sergeant Jack Russell, saw a "huge sea serpent with a horse-like head" in the Georgia Strait. Later, "with a pair of binoculars, Sgt. Russell saw that the strange apparition was a huge bull sea lion leading a herd of six sea lions... Their undulations as they swam appeared to form a continuous body, with parts showing at intervals as they surfaced and dived. To the naked eye, the sight perfectly impersonated a sea monster."[clarification needed]
Some suggestions have been made that Caddy could be an example of the king of herrings or giant oarfish (Regalecus glesne).[citation needed] This species can reach 11 m (36 feet) in length and weigh up to 272 kg (600 pounds). In the opinion of some marine biologists, these long, silvery fish could be mistaken for a sea serpent given the writhing motions they make as they swim.
Decomposing basking sharks may resemble a "pseudo-plesiosaur carcass", and a similar creature caught off the coast of Japan in 1977 was found to be a dead basking shark.
A native image that fits Caddy's description has been traditionally used throughout Alaska. The image indicates that Caddy or a Caddy-like creature moves north from Vancouver when the waters warm. The Inuit of Alaska have even put the picture on their canoes to keep the creature away. The Cadborosaurus is called hiyitl'iik by the Manhousaht people who live on Sydney Inlet, t'chain-ko in Sechelt mythology, and numkse lee kwala by the K'ómoks band of Vancouver Island. However, it has also been argued that these Native stories reflect a diversity of different animals (some mythical, some real) and to prematurely conflate them with Cadborosaurus ignores their diverse origins and often independent backgrounds.
There have been more than 300 claimed sightings during the past 200 years, including Deep Cove in Saanich Inlet, and Island View Beach, both of which are also on the Saanich Peninsula, and at San Francisco Bay, California.
In 2009, fisherman Kelly Nash purportedly filmed several minutes of footage featuring ten to fifteen (including young) creatures in Nushagak Bay. In 2011, a very short segment of the footage was shown on the Discovery TV show Hilstranded, where the Hilstrand brothers (from Deadliest Catch) apparently saw Nash's footage and unsuccessfully attempted to find one of the creatures.