"ce sont" is already the plural form of "c'est"

"Ceux" by itself means "the ones" and it would be used in a sentence like: "ce sont ceux de mon père" = "They are my father's". This is the masculine, plural form of the word, with "celui", "celle", and "celles", being the masculine, singular; the feminine, singular; and the feminine, plural, respectively. When you append "-ci" or "-là" to any word, really, it implies whether the object is near or far to the speaker/situation, and the reason why "-là" is appended here is because "ceux" doesn't have any location associated with, so if you want "ceux" to mean "those" and not just "the ones", then you need "ceux-là". I would prefer to use "ce sont" here instead of "ceux-là" because it's more common and you only really need to add the "-là" if it's important that the listener know where the things are. If you know that "ce" doesn't have any location associated with it and just means "it" or "they" (as in just a placeholder pronoun, if you will), then you will know why "ceci" and "cela" exist (the same thing I just talked about above, but minus the hyphen)

Ce sont les chevaux = Those are the horses.

It should also be accepted, use the "report a problem" feature if you come across this exercise again.

Google translate is very bad at doing its original job (aka translating). It uses literal translation or incorrect vocabulary compared to the context, etc...

However, you can rely on Google Translate for pronunciation (in French at least), as I find it better than Duolingo's for now.

Even better is www.forvo.com which provides recordings from native speakers for many words in many languages.

To answer your question, "Ceux sont des chevaux " is incorrect, but "Ceux-ci sont des chevaux" is correct.

"Il y a des chevaux." = "There are horses."

"those/these" is used either as a demonstrative adjective (in which case we use "ces"):

• "Those horses are happy" = "Ces chevaux sont contents."

or it's used as a demonstrative pronoun (in which case we use "ceux-ci/ceux-là" or "celles-ci/celles-là":

• "I have many horses, those are happy." = "J'ai de nombreux chevaux, ceux-là sont contents."

But sometimes English uses "these/those" instead of "they", even when the object of the conversation is clearly understood. For example:

• "I found keys on the ground." "Yes, they're mine." (but sometimes people might say "Yes, these are mine.")

In this situation, French doesn't really care either, but since "they're" is "ce sont", that's why sometimes you can see "these/those" translated with "ce sont":

• "J'ai trouvé des clefs par terre." "Oui, ce sont les miennes." (mostly you'll never hear someone reply "Oui, celles-ci sont les miennes" unless he/she needs to point at it to avoid confusion, but it's still correct French either way)

I hope this makes sense.

It's just a non-specific article. "Les chevaux" = "the horses" (the particular horses). " des chevaux" = "horses" (some/any horses in general)