Of course we're cheating a bit with this one. Lekku are the so-called fleshy "head-tails" found in Twi'leks and Togruta species of the Star Wars universe. They are highly sensitive to touch, containing unprotected brain tissue. Lekku are the seat of some motor functions and repressed memories. Since the Twi'lek's frequently apply tattoos and glitter to their lekku, and they are highly sensitive to touch, it is reasonable to assume that they are also part of sexual display and an erogenous zone.
Any human-derived clade might have lekku as part of the extensive body modification that humans have undergone as they colonized other systems. As well as the common hairless and fleshy lekku, depending on the specific integument of the clade, there might be hirsute or even scaly or warty varieties lekku with all kinds of interesting textures and fur patterns, including stripes, spots, and colors.
Here are a few other ways to use lekku in the Strange Stars setting:
- Minga pleasure slaves might have these structures for an additional element of body-exoticism. Perhaps they have other functions of which their masters are unaware, such as secretory cells that produce microquantities of substances that increase the target's susceptability to suggestion.
- Some infiltration and shock troops among the Sisterhood of Morrgna might sport lekku that are all muscle: these head-tails allow for deadly whip strikes.
- Lekku would look very handsome on the green-skinned Smaragdine! As a criminally-inclined psi-sensitive species, the additional brain tissue could also allow tricksters among their kind to develop greater psychic potency and/or more specialized psionic abilities.
- The Phantastists could readily create special gene mods for clients to promote lekku growth. The additional brain tissue could increase the client's receptivity to the dream-drugs that are the Phantastists' core business.
- Vokun elders might develop these structures for additional fat and memory storage. The additional brain tissue may be where grudge-memories get deposited, although it is probably also the site of some of the runaway hormonal changes associated with their girthy senesence.