"The horses are animals."
Translation:Is ainmhithe iad na capaill.
This is how the copula works, I'm afraid. Is ... mé/tú/etc. It sort of works out literally as "They're animals, the horses," which I know isn't exactly consoling.
Some other examples: "He is a handsome man." Is fear dathúil é. "Seán is a handsome man." Is fear dathúil é Seán.
"She is a smart woman." Is bean cliste í. "My mother is a smart woman." Is bean cliste í mo mháthair.
I thiiiiiiink I'm right about this, but I may possibly be wrong. Sorry to hedge my bets like that, but I hope this helps!
Also see this comment, it's far more thorough than mine: https://www.duolingo.com/comment/4300985
for what I've understood "Is ...something...mé/tù/sé/sì/muid/sibh/siad " is a defined form, as it is whole a conjugation of the Copula. (I think there's something similar in English for "It's me" where the logical subject is me but the grammatical subject is it).
In other words.. I think you can't use Is without the pronoun, so the logical subject comes after that. Of course it is necessary for third person but rare with first and second (where the subject usually correspond with the pronoun itself)...
PS: I'm sorry for my bad English: I'm Italian and I don't know if what I understand is right nor if what I tried to explain is comprehensible :P
When "i" is used with a possessive pronoun like that, it indicates status or function or condition - "nuair a bhí mé i mo leanbh", - "when I was a child", "tá sí ina máthair mhaith" - "she is a good mother". You can use it to indicate the rank or status that a person currently holds - "tá sé ina mhúinteoir" - "he is a teacher". In that case you are classifying an individual, or identifiable group, based on a characteristic, a characteristic that sets him/them apart from other otherwise similar people or groups.
Being an animal is intrinsic for all horses, so you wouldn't use this structure. You could say "bhí siad ina n-ainmhithe", about an unruly bunch of footballers ("there were animals"), or even "tá na capaill sin ina n-ainmhithe feirme" - "those horses are farm animals".