How about, "that horse is my aunt's" ???
Y. As native speaker this is how I would write it..
'That is my aunt's horse' doesn't pop up as correct answer. Instead, duolingo accepts 'this is my a horse of my aunt'
That is my aunt's horse. is the main answer, and we cannot make it any mainer than it already is. The other problem has been fixed. Thanks.
That is my aunt's horse.