Just a rough idea. I read there's a Chinese mythological creature called Shared-Wing Bird, and that it's born with 1 eye, 1 wing, and 2 legs. They must pair up to fly, and they pair for life. Has affinity with water element, is black and red, resembles pygmy goose. Once paired, if one dies, the other die as well, and I think they reborn together (that part is not so sure). So maybe a married couple can pair their 1-wing bird together and that becomes a 2-seated marriage mount.