In my language (Croatian), to call someone "a horse" (konj, i.e. 'konje' in the vocative case) is offensive and disrespectful, used especially when one's angry at someone who has done something stupid. So, this would work in our environment, as a defensive reply or something. :D Yet, we do consider the horse as the noble animal, ironically.
As far as I can tell, in Arabic, you can use pretty much any animal other than human as an insult.
In Ireland, you might hear someone say "he's a horse of a man" about someone who is strong, or who can keep doing physical work when others would have to stop to rest. You might also hear the greeting "Howya, Horse" between (male) friends sometimes.
In my line of work, we use the horse analogy pretty often. I get where you're coming from.
I don't know why you were downvoted. Additionally, I think Dog (kelb) is probably the most insulting (or at least common) in Arabic.
Maybe they are "undetached horse parts," but they sure look like horses; eh, Quine?
Would "Ni capaill siad" also be a correct way to say "They are not horses"? (I'm on a laptop, so I can't put the accent on the "i".)
No. You only use siad when you are using it as the subject of a verb. You use iad with the copula.