this may be the last one for a bit as i'm trying to get artfight ref sheets done but i'll rapid fire the species sheets and then do their culture planet etc bullsh after everybody's designed. here be wormies in meat suits.
'Unicorn' is a term for the sapient coalition of two species from a temperate planet named Ereshin, large advanced endoparasites named jockeyweaves and a mammalian-similar cursorial hexapod predator dubbed "arda" in the common language (as thats just what they're called in most prominent unicorn languages). Despite the early shudderings about being overtaken by an alien brain worm, jockeyweaves can only sustain life in species from their own planet.
Unicorns are obligate carnivores like arda, utilizing their jaws for hunting rather than their horns as to not risk it potentially fracturing (they're not that fragile but it can happen and it completely disables an entire physical system). Their short intestinal tracts can't quite process plant matter.
Their host species is endothermic and viviparous (placental) but lack the crucial milk production feature to classify them as a mammal (arda foals are born like immediately eating meat and fed via regurgitation).
Their signature forehead horn is a keratinized ovipositor. They essentially reproduce by injecting an arda mare with an agent via this horn that turns it into a passive carrier, and all young this carrier mare foals will become active endoparasited carriers - the unicorns themselves. Unicorns are visually distinct from arda as they genetically alter their hosts still in utero, mostly to the extent of growing the secondary legs into manipulators and setting the path to their eventual horn growth in their "teenager" (prothorate) stage. Arda are herd animals, but not sapient, only the parasite possesses sophont-level intelligence. They are farmed and basically kept as host livestock by unicorns.
Each jockeyweave has one designated host, and cannot survive for long outside of it, but arda tend to be selectively bred for environmental advantage, or simply show. These breeds are their equivalent to ethnicities.
Due to their lack of sexual reproduction and identical/androgynous reproductive organs they don't have much of a concept of sexes/gender, but possess distinct groups from the sexual dimorphism of arda - unicorns with male hosts are cobs, and those with female hosts are pens. There is no difference between them aside from appearance (host genitals reduce and become vestigial), but due to size/strength advantage and the patriarchy instincts blending in from their host species cobs tend to hold authority over pens. They have more access to political positions and in some places reproductive rights, and pens are expected to be meeker atleast in the presence of their bigger, spikier counterpart.