No Gaelic word translates to there in this sentence.
It’s English that’s weird and needs it in this case. ;-) Instead of saying a horse is in X, one rather says there is a horse in X in English. Both sentences mean the same thing, a horse exists and it is located in X. Or horses are in Eriskay vs there are horses in Eriskay, both convey the same meaning, but the latter is more common and natural.
English avoids indefinite nouns as subjects of sentences describing existence of something in a given place, so instead of saying a hair is in my soup you say there is a hair in my soup, instead of starting a story with
in a hole in the ground a hobbit lived you start it with In a hole in the ground there lived a hobbit, etc. Many other languages, including Gaelic, don’t have such structure and don’t need this there.
Because in English horses are in Eriskay wouldn’t be a naturally sounding sentence. You state that thing with there are horses in Eriskay in English. Doesn’t change the fact that the word there here doesn’t really have any meaning on its own and just serves a grammatical purpose – to move the indefinite subject horses after the verb.
EDIT: see this – https://en.wikipedia.org/wiki/Existential_clause#Formation
Many languages form existential clauses without any particular marker, simply using forms of the normal copula verb (the equivalent of English be), the subject being the noun (phrase) referring to the thing whose existence is asserted. For example, in Finnish, the sentence Pihalla on poikia, meaning "There are boys in the yard", is literally "On the yard is boys".
Gaelic is like that – it just states that an indefinite subject (like horses) is somewhere: just horses are in Eriskay, it means that there exist some horses and that they are in Eriskay.
In English, on the other hand you use the dummy subject pronoun there in such cases:
In English, existential clauses usually use the dummy subject construction (also known as expletive) with there, as in "There are boys in the yard"
In some other related languages different pronouns or even verbs are used, eg. in Norwegian you may say det finnes en mann… for there is a man but it literally translates to something like it is found a man…. In German you could say es gibt einen Mann…, lit. it gives a man…
Gaelic tha eich ann an Èirisgeidh and English there are horses in Eriskay both mean the same thing and both are grammatical sentences in their respective languages. That’s why the correct answer uses this phrasing in English.