How to Survive a Mermaid Attack, 1527

British Library MS Royal 2 B VII, f. 97r

The mermayde is a dedely beste that bringeth a man gladly to dethe. Frome the navyll up she is lyke a woman with a dredfull face, longe slymye here a grete body & is lyke the egle in the nether parte, havinge fete and talentis to tear asonder suche as she geteth. Her tayl is scaled like a fisshe and she singeth a maner of swete song and therwith deceyveth many a gode mariner for whan they here it they fall on slepe commonly & than she commeth and draweth them out of the shippe and tereth them asonder... but the wyse maryners stoppe their eares whan they se her for whan she playth on the water all they be in fear & than they cast out an empty tonne to let her play with it tyll they be past her. This is specifyed of them that have sene it.  

Laurence Andrew, The Noble Lyfe and Natures of Man of Bestes, Serpentys, Fowles and Fisshes

The bad news is that the slimy-haired mermaid of dread wants to tear you asunder with her talons. The good news is that you can distract her with an empty barrel.

How to Avoid Bad Weather, 1660

Edward Topsell, The History of Four-Footed Beasts and Serpents (1658)

"Whosoever are defended by a skin of a River Horse called Hippotamus, are never touched by the Thunder... some say that if you hold a Looking-Glass against the Cloud that hangs over you, the Hayl will pass away:  also if you compass the place with the skin of an Hyæna, Crocodile, or Sea Calf, and hang the same up before the Doors of your house, for then the Hail will not fall." 

Johann Jacob Wecker, Eighteen Books of the Secrets of Art & Nature

Added bonus: nothing says "welcome!" like a crocodile skin nailed to your front door.

How to Whiten Your Teeth, 1607

That Unicornu fossile is definitely legit.
Michael Bernhard Valentini, Museum museorum (1714), George Peabody Library.

"The horne of an Vnicorne being beaten and boyled in wine, hath a wonderful effect in making the teeth white or cleare, the mouth being well clensed therewith." 

Edward Topsell, The Historie of Foure-Footed Beastes (1607)

Wine and unicorns really do make everything better, don't they?