Beithir

No single canonical origin is recorded in the provided sources. Folkloric motifs and scholarly suggestions place the beithir among the fuath (water-associated monsters) of Scottish tradition, and link its narratives to wider motifs: tales of reconstitution (a killed snake whose parts rejoin becoming a beithir), accounts of large beasts roaming to lochs (e.g., the Scanlastle/Loch-in-daal episode recorded by John Gregorson Campbell), and interpretive comparisons by folklorists (Donald A. Mackenzie) to figures such as the Cailleach Bheur. Etymological discussion (MacKillop cited) also suggests uncertain ties between senses of 'serpent', 'wild beast', and possible Norse influence in the word's history; these points are presented in the sources as suggestions rather than a single origin narrative.

Sources describe the beithir as a very large snakelike creature or dragon but often explicitly note the absence of some European draconic traits (no wings or fiery breath). It is characterized as the 'largest and most deadly kind of serpent' in accounts and sometimes appears in tales as a 'loathly worm' or in enchanted form (for example, a magic garment in John Francis Campbell's tale that is described as a beithir in disguise).

Folklore attributes to the beithir a venomous sting that can rapidly endanger a person; tradition prescribes reaching water as an immediate cure. It is described as predatory and destructive in local accounts (one reported to have devoured seven horses on its way to a loch). A recurring motif is reconstitution: if an ordinary snake is killed but its head is not separated and destroyed properly from the body, the parts will rejoin and the snake will return to life as a beithir. Scholarly commentary (E. C. Watson) and etymology suggest an interpretive link between the beithir and destructive natural phenomena such as lightning, but sources present this as an explanatory metaphor rather than a documented supernatural power of controlling lightning.