Goblins & Gremlins

Goblins, in much art from the Olden Days, are not the small green-faced guys we now recognize. Just as often they're small noblemen like Rumpelstiltskin, cat-faced critters in childrens' clothes like in Rackham's illustrations, or etc.

The modern goblin (in D&D) is different than these because it's a) straight from Tolkein, and b) not a fey creature any longer, just a natural little hominid. The stuff that would have been goblins and gremlins and the like are now redcaps, piskies, changelings, etc.

If we want to bring back the 'wonder' of them we can really just draw a distinction between the True Goblins and everyone else, but note that the common folk often lump everything together as "goblin" in their speech. Some things are clearly Not Goblin: dragons, shambling mounds, and yuan-ti aren't goblins. However, ratfolk, kenku, gremlins, pixies, grigs, any wee beasties, actual goblins, kobolds, redcaps, and etc.: all goblin. Basically any non-humanoid under 3 ft tall is probably goblin, especially if it has animal features.