This pencil drawing has got to be the only illustration of a Fremlin ever made, because its all google can find. But its the exact illustration I remember from the 2nd ed AD&D Complete Book of Humanoids, so its good enough for me.
In any case, Fremlins are weird and ridiculous. At first glance you think, "Oh, its a gremlin with wings. Not really exciting." But regardless of whether or not you are a fan of that one bat-gremlin in Gremlins 2, these guys have a lot of things going for them. However combat capability is not one of those things. They tend to be cowardly, lazy, whiny, and plump little dudes who run from combat, and they hate using weapons; but who just also happen to be IMMUNE TO NON-MAGICAL WEAPONS. Holy crap. Why are they immune to non-magical weapons? No one knows, they just are. And remember these guys are in the Complete Book of Humanoids, so they are a playable race in AD&D, so of course we had a buddy who played one to great comic effect. Immune to non-magical weapons at level 1? Yes please.
One last thing that makes me laugh about these guys is that their description also points out that they "shun clothing and ornamentation." And if they are part of your adventuring party that means they likely tend to hover around head-height so they can communicate more easily with all the normal-sized humanoids. But they're nude all the time, so its just floating Fremlin dong wherever you look. Yeesh.