Biasd-na-sgrogaig (also variously biast-na-sgrogaig, beisd-na-scrogaig etc.) is a type of an aquatic unicorn peculiar to the Isle of Skye and generally the Outer Hebrides of Scotland. It's been compared with a huge ram or a shire horse. The creature is clumsy and inelegant, tall and awkward, with very long legs and a single towering horn. It lives in certain sea lochs. John Campbell believes it to be a bogey invented by parents to make their children quiet. Alexander Forbes sees the origin of the legend in a narwhal that could have strayed south from the Arctic seas. Certain dictionaries mention usage of the term to refer to the heraldic unicorn.
In Scottish Gaelic "béist" or "biast" means as much as "monster"; the former term being stronger than the latter. "Sgrogag" is a humorous term for a horn, especially small or crumpled – "sgrogaig" is the genitive form. The whole name can be understood as "monster of the horn".
One source, unfortunately not scholarly, spells the name of the Scottish unicorn as baiste-na-scoghaigh. According to it, all of its race are male and it mates by taking on human form and copulating with a human woman, siring only sons. The creature cannot tolerate weak men, whom it regards as an affront to its own virility, and is likely to try to dispatch them with its fearsome horn and take their place aiming to sire a more worthy male son.