"Horses are animals."
Translation:Is ainmhithe iad capaill.
I think that the pronoun is integral part of the Copula: "Is - subject complement - pronoun - logical subj (if needed)" is like a conjungation in Standard Irish.
There's a complete explanation with several examples here: https://www.duolingo.com/comment/4300985
For a comparison of the Copula between English and most of other languages: http://en.wikipedia.org/wiki/Copula_(linguistics)
I agree, if you read this section in GnaG (I think that it is really the same thing, with plurals instead of singulars), the iad in this sentence seems just plain wrong. It is a classificatory sentence with two indefinite nouns, no iad is needed. In fact, GnaG states that such sentences with an indefinite subject are uncommon, and that a sentence with a definite subject would be preferred in this case, so Is ainmhithe iad na capaill. (just like here). There is no mention of a dialectal variation that would use Is ainmhithe iad capaill.