these are my handsome horses is also correct! I sent this to my German friend and he agrees that it is, and that it is stupid to mark it wrong. This same word is used for Hallo Schoner...hello handsome. So why not for horses. Horses can be handsome...actually it is more appropriate.
For heaven's sake Duo, does everything have to be beautiful or individual? Can we have some red stuff and some cold stuff, or any other adjective that doesn't have 300 letters in it? I'm on level 4 of the last of the adjective declension units and my fingers are getting tired!
Applying the rules of http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html (easier than memorising the tables): it's attributive, it has an article, the article has nominative form, the article is plural; therefore it takes -en.
I agree that the ending is -en for the plular (I wrongly thought that in nominative it is different). Anyways, both adjectives should be in the same form : "meinen schönen Pferden". Not only my teacher told me this, I also found it on web pages about declining german adjectives in my native language.
The mistake here is that mein is not an adjective, But a determiner (like an article). They decline differently to adjectives.
What your teacher meant about "all adjectives getting the same ending" is that you could write for example Das sind meine schönen großen braunen arabischen Pferde.