Translation:This girl does not like the grey horse.
Because (не) нравятся requires the dative case for the one who likes and the nominative case for whatever is(n't) liked.
The gray horse is the subject in this sentence and the girl is the indirect object.
It's a very unusual sentence. The only time someone would say this is if they were trying to exaggerate, e.g., "this girl doesn't like a single grey horse in the world," which puts lots of emphasis on the sentence and makes it seem either exasperated or braggadocious, and without that emphasis, it sounds extremely strange. Otherwise, if they weren't specifically singling out a horse, most people would just say "this girl doesn't like grey horses," and that would cover the general case.
And would "this girl doesn't like grey horses" also be rendered via the plural по русски, как "не нравятся серые лошади" ?
You guys are confused about cases and why the ending of word changes. Cases use to indicate how a noun is being used. And they change the ending to represents its value. For example. These are all the cases. [[Nominativ=subject (example-мама=mom)]] [[Genitive= origin/from/of (example-мамы=from mom]] Dative=To/indirect (example-маме= To mom]] Instrumental=with/by (example- мамой=With mom]] Prepositional=on/about (example-маме=on mom]] remember that this is how the noun would look like if they used on a specific sentence.
Dative. Think of нравится as 'is pleasing to'. The horse is in the nominative because it is doing the pleasing; it is technically the subject of the sentence. And the horse is pleasing to the girl. The 'to' there makes the girl the indirect object of the sentence, putting her in the dative.
I have been using Reversio to help with meanings. It suggests 'the' as an option for этой but it wasn't allowed should i report it or stop using Reversio?
You should report it. It's not that unusual for это to translate as "the".