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. 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.
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. They are the anti-gnomes.
In Dungeons & Dragons, at least in AD&D, gremlins are described as a member of the goblinoid family that resemble imps. Fiend Folio didn't define them but did describe several monsters either as gremlins (galltrit) or as jermlaine-related (mite aka pestie, then snyad). Tom Moldvay brought them into the "unadvanced" line through X2: Castle Amber. Thence they entered the Companion Set where, as usual, they appear out of alphabetic order. They wouldn't have it any other way, we suppose.
Second Edition was the heyday of this genus. By 1993 the Monstrous Manual had for the Gremlin category five different kinds: the standard gremlin, the Fremlin (Friendly gremlin), and the aforementioned Galltrit, Mite, and Snyad. For whatever reason the Jermlaine were NOT so annexed; they're listed on their own as "Gremlin, Jermlaine". Also this Manual claimed all the big five have wings and small tails, which had not been true of mites and snyads up to now (especially since these live in tiny underground passageways, like the jermlaine); so the "mite" entry has to go backsies on that. Since a gremlin was running TSR at the time, arguably she'd know best.
WotC was slow to bring back any of these for Third Edition, only including the Jermlaine and that had to await the second Monster Manual volume. (Why not the 3e Fiend Folio? Dunno!)
Necromancer jumped into the gap, and for Tome of Horrors included the Mite again. Although, this has them as goblinoid (perhaps to avoid the 2e classifications) and disassociates the pestie into a subtype. The third Tome volume has a separate entry on gremlins. Obviously none of this is D&D canon.
Various gremlinkind debuted 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. They've shown up in various Bestiaries.
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The gremlins in That Movie reproduce asexually, either directly or else from mogwai. In the sequel, which was a sight better than the original, several full-fledged gremlins break into a bio lab. Relevant here, one takes a massive shot of hormones. Where they had been agents of pure spiteful destruction, which would mostly be coded "male"; this one went femme.
Oh yeah. Gremlin monstergirls are a thing.