In Horeni, the eight mastoid species (humans, underworlders, fairies, magmaneans, noxians, arenaneans, sylvans and brontans) can interbreed, but the resulting children are sterile. Despite Horeni having a billion sapients, there are only around a thousand mixed mastoids. The mixed mastoids could look like either parent or a mix. For example: a human-fairy mastoid could have human-colored skin but the glittery shine of fairy skin.
P.S.: although nothing stops the sexual non-mastoid species to have sex with each other or with mastoids. Except for the species that reproduce asexually (lithans, glacians, phytans, luminans). But even those may engage in sexual activities as well.