Matunus or Matunos was a god in Brythonic

Britons (historical)

The Britons were the Celtic people culturally dominating Great Britain from the Iron Age through the Early Middle Ages. They spoke the Insular Celtic language known as British or Brythonic...

 Celtic polytheism

Celtic polytheism

Celtic polytheism, commonly known as Celtic paganism, refers to the religious beliefs and practices adhered to by the Iron Age peoples of Western Europe now known as the Celts, roughly between 500 BCE and 500 CE, spanning the La Tène period and the Roman era, and in the case of the Insular Celts...

. His name may be derived from the same root as Proto-Celtic *matu- meaning bear

Bear

Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

. He was worshipped in Roman Britain

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

 and altar-stones raised to him have been recovered in the United Kingdom, such as at High Rochester (1265 [AD213]) and at Risingham.

The god may be parallel with Mercury Artaius

Artaios

Artaius is a Celtic epithet applied to the Roman god Mercury during the Romano-Celtic period. It is known from a single inscription from Beaucroissant in the Isère:...

, who might also have ursine connotations.
A similarly named Gaulish god

Gallo-Roman religion

Gallo-Roman religion was a fusion of the traditional religious practices of the Gauls, who were originally Celtic speakers, and the Roman and Hellenistic religions introduced to the region under Roman Imperial rule. It was the result of selective acculturation....

, Matutinus, is attested in at least three inscriptions from Switzerland; in all three he is identified with Mercury

Mercury (mythology)

Mercury was a messenger who wore winged sandals, and a god of trade, the son of Maia Maiestas and Jupiter in Roman mythology. His name is related to the Latin word merx , mercari , and merces...

, and in one he is also identified with Cissonius

Cissonius

Cissonius was an ancient Gaulish god. After Visucius, Cissonius was the most common name of the Gaulish Mercury; around seventeen inscriptions dedicated to him extend from France and Southern Germany into Switzerland....

.