English words (unlike Chinese words) usually do not require a measure-word or quantifier — this is because most English nouns are countable (on their own). On the other hand, singular countable nouns always require a determiner (like “a(n)”, “the”, “this”, “that”, or a possessive). In this case, the best way to resolve the issue is to make the subject plural. The grammatically correct sentence would be: Crocodiles are vicious animals. In the singular, we’d need an article — either “a” or “the” would work here, with almost exactly the same meaning (in most contexts). A crocodile is a vicious animal. We could use "crocodile" without a determiner in reference to the meat (which is uncountable): Crocodile is delicious.

Hi! It should be 'crocodile is a vicious kind of animal'. the word "vicious" is an adjective and it describes the noun "kind". Again, the correct one is: "Crocodile is a vicious kind of animal."

This can be a bit confusing. If you use KIND OF before an adjective, it means SOMEWHAT. Ex: The test wasn't as hard as the one last year, but it was still kind of difficult.

Actually they are both wrong, because we wouldn't use "kind of" here. If I ask "what kind of animal is a crocodile?" then the answer I expect is "it's an amphibian". You might say "the vicious kind!" as a joke, because we understand "kind" to mean a specific group, not just a characteristic. So "kind" is a term that stands in for species, breed, type, make, shape... The best way to say what you want is probably "Crocodiles are vicious animals".

My pleasure!