Not just a frog...

 

Faeries often disguise themselves as toads, frogs or even mice, which can be dangerous as frogs and toads especially are often used in magic spells or  cures.

The bones of a green frog, the flesh having been consumed by ants are thrown into fresh water. Some will sink and some will float. Take the bones that sink, hang them up in a fresh linen cloth. These will bring love but beware if touched they will bring hate.
This is taken from The Discovery of Witchcraft written in 1584

Frogs are also used to cure thrush: take a living frog and hold it in a cloth, put the head of the frog into the patients mouth making sure that it does not go down the throat. Hold it there until the frog is dead and then take another frog and do the same.

For a child that wets the bed, roast a mouse and give him the gravy to drink, and it will cure it or for small pox eat a fried mouse.

These are traditional cures from the fifteenth and sixteenth centuries so I am not advising that you should try them! And if they were really faeries in disguise I think they would be a little bit annoyed and remember never piss off a faerie!