Why is 'Ini kuda yang tercepat mereka' wrong?
I think "kuda" and "mereka" have to stand next to each other to clarify that it is "their (fastest) horse"
Could you switche the position of kuda and ini
If you switch these two it would mean "This horse of theirs is the fastest" (in the city, or in the world or whatever). But the meaning here is that the horse is the fastest of all the horse they own.