Something to do with 'hearing' not being a correct active verb in speech. You would normally say "I hear an animal" and not "I am hearing an animal." I don't know the technical reason, but "I hear an animal" sounds better and is the correct one.
Maybe someone else can get a little more in depth.
Should be accepted. They are really interchangeable in English, although a lot of English language learners will dispute me as there is often a grammatical equivalent of "hearing" (using the gerund) in other languages. As a native speaker and a long time teacher of English as a second or foreign language, I can tell you, you were right. :P
There is a difference between action verbs and stative verbs. Normally, stative verbs are not used in a continuous sense--only action verbs do that. Most verbs are action verbs, a few are stative verbs, and some can be one or the other depending on how it's used. Verbs showing senses (hear, smell, see, feel) are among the stative verbs. https://www.thoughtco.com/differences-between-action-and-stative-verbs-1211141