"On whose horse?"
Translation:Op wiens paard?
Ja, dat had ik ook, op wie zijn paard hoort ook goedgekeurd te worden.
"op HET paard van wie?" would be possible if you'd put a lot of emphasis on wie. It would sound as if you were quite surprised or upset regarding the owner of the horse. DL suggestion is more neutral and formal.
That sounds pretty old-fashioned to me, it would immediately strike me as odd if anyone would use it. Also your link also mentions "dated". But maybe it is still commonly used in some regions/dialects?
On second thought, I am inclined to agree.
I think it is a shame 'wier' is in so little use today. I for one use it. It just adds this little bit of extra colour to the message.
I fear the advantages of accepting 'wier' (some extra understanding,aesthetic, ...) do not outweigh the disadvantages (added complexity, datedness, ...), here on Duolingo.
If there are dialects or regions in which it is in active use, I do not know them, but then again I do not know all that many dialects anyway.