A xenomorph is a pretty good choice for a knuckle duster topper I think. According to Wikipedia, these types of Aliens, unlike many others pitted against humans in the sci-fi genre, "do not have a technological civilization, but are predatory creatures with no higher goals than the propagation of their species and the destruction of life that could pose a threat." And isn't that also the basic sentiment of anyone hurling a set of brass knuckles at someone else's face? Xenomorph, fist weapon, a logical match.
This particular xenomorph knuckle duster has the added bonus of folding into itself such that a wearer can also sport it as a less confrontational ring. Handmade to order by Kilroy of Kilroy's Attic, the stylized transformer materializes from pressure-cast resin infused with a stainless steel powder coating and 30 strands of 20-lb test line interwoven at each hinge. Kilroy deems the ring a "powerful piece of alien tech."
The xenomorph head detaches to unfold the ring, and then reaffixes to lock its four-finger knuckle duster in place; it sits inside the palm in that formation. Fully unfurled, the piece measures 5-1/4" long x 2" tall. The xenomorph itself is 1-5/8" tall x 2" long x 3/4" wide. Ring sizes begin at 6 and go up to 11.25.