Translation:There are twenty one horses on the farm.
Even after reviewing how numbers work, I'm completely not seeing the "one" in twenty-one. Is this not "there are twenty horses"?
Looking at the example at https://www.duolingo.com/skill/ga/Numbers, "21 cats" is"aon chat is fiche" (one and twenty cats). Is this a mistake, or is there another capall hiding out somewhere? Should this not be, "Tá capall aon is fiche..."?
Not really. Remember, that the number usually comes before the noun. However, aon chat is fiche sounds weird. Gramadach na Gaeilge gives it the same structure that's here, and it's all I've seen from native speakers (fáilte is fiche, for example). Basically, what you're saying here is "a horse and twenty".