In English "is not" in Portuguese not only the words are different also the way of making the sentence too different. I like it.
"It is not my horse" is marked wrong, but this should be the really correct answer, because this is everyday English, whilst "The horse is not mine" sounds a bit stilted.
Pq é "the "horse's" not is mine" pq tem o " 's " ?
Abreviação de "is".
It is not my horse
Exactly. It should be possible to write this as well. "The horse is not mine" sounds weird pragmatically and would almost never occur in real life.