English Noun Grammatical Number has complexity beyond a false generalism; Every Noun has a single Singular and Plural form for which the Plural form takes Plural grammar and represents multiple instances of the Singular.
The Noun word, English, is an example - there is no Englishes Plural form. There is Old, Middle, Medieval, Provincial, Colonial, Contemporary, Modern, Spanglish, Oxford, British, English dialects, accents, & etc. Despite the billions of ways humanity and some other animals use English, the Noun takes Singular form and Verbs for the Noun conjugate with Singular Number.
Invariant, zero-plural, irregular, collective, mass, countable, uncountable, Plural form uses Singular Grammar, double Plurals - different meanings
See here for explanation. https://www.duolingo.com/comment/3734833/Grammar-niet-and-geen
The problem with this sentence is the unclear (maybe even impossible) translation to and fro. The dutch know: 'Het zijn geen schapen' and 'Ze zijn geen schapen'. Using 'het', one refers to the animal; using 'ze' one refers to a human acting like the animal. Apparently the english translate those two sentences with the same words 'They are not sheep'. Translating this back to dutch, the dutch wil regard 'they' as the personalized form, so: 'Ze zijn geen schapen'. So my dear english-speakers: to avoid misunderstanding when speaking with a dutchie, use 'it' when you mean the animal and 'they' when you try to describe the behaviour of people. 'It ain't sheep' vs. 'They are not sheep'. ;-))