You can use Op het paard van wie?, but it's normally used to have some extra emphasis on wie and/or to express more emotion (you're surprised and have no idea who the person is whose horse will be used). I guess wiens is more common.
Is "wiens" also used in flemish? Sorry for asking, I know it's dutch from the Netherlands that we learn here. But as a belgian, I mostly need to learn flemish. So if a flemish could help me with this one, it would be really cool. Thanks!