"Dude, what's a Gremlin?"
The modern Gremlin seems to have evolved from a variety of mischievous household sprites, in particular the Celtic Boggart and the Irish Pooka. This, along with the species’ name being derived from the Gaelic word Gruaimin (“little guy with a sick sense of humor”) According to Webster’s Dictionary, indicates an Irish origin to the Gremlins.
In any case, Gremlin folklore had its heyday in the British Royal Air Force of World War Two. Rold (Charlie and the Chocolate Factory) Dahl is said to have written a book of Gremlin stories collected during his stint in the R.A.F. Walt Disney once optioned it as a movie but I haven’t been able to find this book anywhere (NOTE: it's currently back in print, published by DarkHorse). In it, he describes them as something vaguely reminiscent of the American Jackalope (a hare with horns), and wearing either bright clothing or a flying jacket. The young are called Widgets, the males are called Bocans, and the females are called Fifinellas.
Gremlins were credited (if credit is the word) with any malfunctions or inexplicable events in the air. Later, the little creatures would be domesticated and be the reason for any odd noises or such.
The second Golden Moment in Gremlin lore takes us to 1984 and some monster movie by Joe Dante and Steven Speilburg. Go figure, I’m not a terribly big fan of the “Gremlins” movies. However, to be fair, information on them can be found here, here, and also here.
There are several different types of Gremlins. A subspecies who can operate above 10,000 feet is called a Spandule. A spacefaring Gremlin is called a Galactic Ghoul, and these are said to be native to Mars. The Tree-Squeak of camper’s folklore is often classified as a sort of forest Gremlin. The word Flibbertigibbet is occasionally used as the name of the offspring of a Gremlin and a Fire elemental, and there are aquatic Gremlins who attack boats. There’s also a group of Hawaiian “little people” called the Menihuni, who are occasionally classified as a kind of forest Gremlin. They are very laid back and live behind waterfalls and are said to be the most humanoid looking.