The word ‘sheep’ remains the same in the plural and singular form; the same form of the word could be used in both situations:
Singular: The sheep was kept as a pet by Tommy.
Plural: The sheep looked funny after their thick coats of fur were shaved off.
While it’s true that most words in the English language can be transformed into their plural form by simply adding an ‘s’ at the end - like ‘bird’ would turn into ‘birds’, ‘basket’ would become ‘baskets’ and so on - it is also the case that some other words adopt the same form as both singulars and plurals; for instance, the singular word ‘trousers’ remains the same as a plural, and so does the word ‘scissors’.
(It must be noted at the same time that not all words change themselves into plurals simply with addition of ‘s’, there are different ways of doing it and one must be cautious of that.).