Just got a Duolingo feedback email. "Voulez-vous monter le poney" should now be an acceptable translation
And still not fixed! Voulez-vous Vs Veux-tu = formal Vs informal
"Tu veux faire du poney ?" is an accepted alternative.
(I was half expecting to get dinged.)
I think I have seen here the suggested translation of poney as le petit cheval
I'm a bit confused... Larousse says that To ride [a horse] is "Monter à..."
So why isn't it "Veux-tu monter au poney"?
Interestingly, only "monter à poney" (to ride a pony or to ride ponies) and "monter le poney" (to ride the pony) are grammatically correct. "Monter au poney" is not correct. I don't know how else to explain this peculiarity.
Sur le poney