Why is उसके translated as she and not as they? Isn't उसके the plural form? Shouldn't it be उसकी instead?
उसका/उसकी/उसके translates with the object, and not the subject. As well, when words like पास or बीच or सामने are used it always takes the उसके form. In fact, उस can be either a boy or a girl. We have no way of knowing which from this sentence.
So उसकी बहन है could be 'She has a sister' or 'He has a sister'.
Similarly उसका भाई है could be 'She has a brother' or 'He has a brother'.
And उसके दोस्त हैं could be 'She has friends' or 'He has friends'.
For a plural subject we use उनका/उनकी/उनके (Their brother, sister, friends, etc.).
So if we cannot know the gender of this sentence, I think it only would be fair to give both he and she as an option to choose from.
So, does the ending of each of those three first words from your message correspond to a gender of the subject? Horse in this case. I thought the third one is for plural, but we have "a horse". I'm confused
It takes the plural form because it is with a postposition, in this case, it is with "pas" पास. I think it's called oblique form when a post position changes it to the plural form.
Of course, now that I said it, I'm wondering if I have my terminology correct. Is this a postposition?