"These are horses."
Translation:Das sind Pferde.
We use neuter singular forms das or dies to introduce something new to the conversation, regardless of the gender or number of the thing(s) you are introducing.
The verb has to match the number, though -- so you can have das sind... if the things are you introducing are multiple things.
When you are introducing something new to the conversation, you use neuter singular terms: das or dies.
This is true regardless of the gender of the new item(s) or even of how many there are -- so dies sind could be "this is" or "these are".
die sind would be something like "these ones are" -- referring not to something new but to a subset of things that you had already spoken about and where you know what kind of noun you are referring to.
For example, if you had spoken about a specific group of animals and now wanted to focus on a particular subset of them and say that "these ones (= these animals) are horses", you could say Diese/Die (= diese Tiere/die Tiere) sind Pferde.
Yes, dies is neuter singular -- that is the form we use to introduce something new, regardless of the gender and number of thing(s) that we are introducing.
Dies ist ein Hund. (masculine), Dies ist eine Katze. (feminine), Dies ist ein Pferd. (neuter), Dies sind Tiere. (plural)
Similarly with Das ist ein Hund. Das ist eine Katze. Das ist ein Pferd. Das sind Tiere.
A correct German sentence, but not a correct translation of the English one.
You wrote "They are horses" rather than "These are horses" -- you used a personal pronoun (referring back to something previously mentioned) rather than a demonstrative pronoun (which can introduce something new).
Context and practice.
"that" in English also has several meanings (e.g. the two different ones in "He said that that horse is fast").
When das is before a noun, it can mean either "the" or "that" -- you can hear the difference in speech (das is stressed when it means "that" but not when it means "the") but you can't see it in writing; then only context can help. (On Duolingo, typically both translations will be acceptable.)
In a context such as Das ist..., it's "that is...", while Das sind ... means "those are ...". (Or Das ist ... = this is ...; Das sind ... = these are ...; since German doesn't distinguish between "that" and "this" so strongly -- but if in doubt, stick to "that" and "those" respectively, choosing between those two depending on the verb and the subject.)
What is wrong with "Diese sind Pferde" as the translation of "These are horses"?
That it's not a translation of that sentence. It means "These ones are horses" -- i.e. referring to a particular sub-group of a specific group you had been discussing before.
If you are introducing a new topic by "pointing" to it with "this, that, these, those", use neuter singular in German: Dies sind Pferde.