It has been three days since you crossed the farthest border of the tribal Orc lands. According to the shaman you interrogated, the kingdom of Jeng-Gu is only 10 days' journey away. As you travel, the ranger notices footprints near the bushes. Some are booted but others are impressions of bare feet. She recognizes the three-toed footprints immediately: Goblins. The party carefully fans and searches, then spot movement near a stream. The warrior carefully approaches a Goblin standing inattentively near the foliage. He prepares a mighty leap to cross the 10 feet or so between him and his mark, a skewering, leaping charge that should lead to a quick surrender. Enough time has passed: the others are in position. He leaps, ready to deliver a killing blow to his enemy's back! He brings his mighty bastard sword down and...there's is a blur of movement and he finds the blow blocked by his enemy who has turned but is still not facing him. It wheels around to face him. The fighter realizes that this thing isn't a Goblin, at least not the normal kind. From its hairless head spring two antennae; its swollen, bulbous eyes are solid black and gleam with hunger; from within its mouth, attached to its upper jaw, a set of mandibles juts out. The ambush turns into a blood-soaked skirmish; the Goblin-things move with perfect, wordless coordination. An unnatural coordination. But the party triumphs; a bloody and pyrrhic victory that might just guarantee their defeat in the next battle. Looting the bodies, the thief finds an ivory amulet with a figure carved upon it: The Goblin god of war, Ramahsa. But there is something amiss with his depiction; his hands are pincer-like claws, his head combines the worst aspects of a mantis, an ant, and a spider. What horrors have you stumbled upon?
Pardon the above piece of overindulgent and overly RPG-recap-esque fiction but I was particularly inspired at the time. If there's one old D&Dism that I actually find charming, it's got be the concept of subraces. I'm not talking about the relatively tame subraces like Hill Dwarves (like Dwarves, but with Hills!) or Wood Elves (like Elves, but with more redneck!) but really crazy stuff like aquatic Gargoyles, the half-plesiosaur Sea (or was it Ocean?) Giants, or the weird monkey Goblins.
In a low-fantasy world, I believe that such really bizarre variations help create a more memorable and adventurous experiment. The places where normal monsters dwell should actually be pretty well-know and close by the core region where the game is taking place. But beyond those monster-ruled wildernesses there should be something it is weird.
In my little fiction, I assume Goblins are roughly on par with an average Human but much more cowardly. But the bizarre Insecto-Goblins the party encounter aren't cowardly and in fact seem to show no fear at all, fighting to the bitter end even when retreat is a sensible idea. There's also hints that these guys have a hivemind or telepathy which gives them their "unnatural" coordination.