Hi Saeiiiid. This question has to do with subject-verb agreement. If the subject and verb are singular, as in "Animal is," then the direct object must reflect this in number. So it would read, "My favorite animal is 'a' horse." If the subject and verb are plural, "My favorite animals are," then the direct object would be the plural "horses" without the qualifying article "a." I hope this helps.

You would say: "Horses are my favorite animal" "Horses" is the thing you're talking about, and "my favorite animal" is describing "horses"

You can say, "My favourite animal is THE horse" or "The horse is my favourite (kind) of animal". (both singular and using THE because you are speaking in general about a group) You can also say, "My favourite animals are horses" or "Horses are my favourite (kind) of animals". (both plural)

Hi Thanks But I saw this sentence in a famous grammar book (practical English usage) by Swan: The biggest problem in my garden is wasps. Is that correct?

NO you can't Because you are talking about horses you have to have subject + verb agreement. You have two choices to put Your favourite/favorite animal as the subject at the beginning of the sentence then agreement comes at the end "My favourite animals are horses"" or second choice "Horses are my favourite animals" horses at front of sentence and agreement at the end. The sentence must be plural at both the beginning and the end. If it is single the same rule apples but this time it must be singular at the beginning and also singular at the end. "My favourite/favorite animal is a horse" "A horse is my favourite animal" . So no you cannot say or write a single and a plural in one sentence or clause.