From 1d4chan
Watch out for this little monster.

Gremlins are a kind of monster originating from Britain "neo-mythology", having been concocted by members of the British Royal Air Force within the 1920s as a goblin-like creature that loves mechanical devices and tinkering - or, more accurately, breaking things, especially to get people hurt.

Up to 1983 the most famous gremlin in popular culture was the one Richard Matheson sicced on William Shatner in the 1961 anthology Alone by Night, very soon a Twilight Zone episode. Jermlaine filled that niche in Advanced Dungeons & Dragons, perhaps from the French. And there was a car. Loosely defined.

Gremlins by that name became famous again in the 1980s, that cultural wasteland, because of the Twilight Zone movie and then a disgustingly bad Stephen Spielberg outing. So of course various roleplaying games had to catch up and include them somewhere. For Dungeons & Dragons they got into the Companion rules where, as usual, they appear out of alphabetic order. They wouldn't have it any other way, we suppose.

Gremlins most commonly appear in dieselpunk or urban fantasy settings, where their status as magical creatures that love to meddle with machines makes most sense. Settings aiming more to fantasy, like D&D noted above, will portray them as a particularly nasty goblin or faerie creature with a particular knack for traps.

Various gremlinkind feature in the Pathfinder adventure path Legacy of Fire, at least for its opening act Howl of the Carrion King - and indeed, in the desert you do not want to meet up with them. They also infest the Darklands.