There are some distinctions here between these variations. I don't know what your original intended communication is. Here is how I read them:
Man is an animal. (Humans are part of the animal kingdom.) Neutral statement of classification
Men are animals. (Male humans behave in uncivilized ways. ) Pejorative statement (in many contexts, especially if delivered by a female human)
Man is animalistic. (Humans behave in many ways just like animals.) Evaluative statement, but not necessarily negative
I think the third sentence implies that we humans behave without regard to our "higher" thinking functions, working on instinct rather than reason. That may just be my interpretation.
Since A / AN was derived from the word ONE (hence N in AN), the sentence
"A man is an animal" actually reads: ONE man is ONE animal.
The word ONE is used in most European languages (even in Polish, although most people do not realize it) to show that the speaker isn't talking about any specific individual or object. When the speaker does refer to someone or something specific, then ONE means simply the same as someONE or something.
Therefore, saying "A man is an animal" is, as JamesM already wrote, a case of classifying things without refferring to ANY specific man and ANY specific animal.
By the way, the word ANY was alos derived from the word ONE.