Etymological excursus: Gaelic

While looking for information on the boobrie (a monstrous black bird of Highland folklore), I found mention of a many-headed sea-creature called the uilbhéist, supposed to protect coastal inlets. I didn't find much online, so I turned to McBain's Gaelic dictionary. Near as I can figure, it looks like a compound of uile "all" (cognate to the English word) and béist "beast" (both from Latin bestia). That is, "all-beast". Given the popularity of merfolk in Celtic folklore, I can't help but speculate the desgination might distinguish those that were "all beast" from those that were part human.

Be that as it may, a couple entries down was a native term for "beast" that the Latin borrowing helped displace: beithir. In Gaelic, th represents [h], so the pronunciation is more like behir. Look familiar, Greyhawkers? Among its meanings are "bear" (due to English influence), "huge skate ray", and "serpent". Not as far off as D&D critters often are!