This is my vision :): a (though it is used in many different situations) in your question can be used to give a definition: ‘What is a donkey?’ - ‘A donkey is a grey or brown animal like a horse, but smaller and with long ears.’
To refer to the whole group you say ‘donkeys’ like in ‘The rugged terrain is inhabited by deer, antelopes and, locals say, wild donkeys.’
In academic or formal language, mainly to describe typical characteristics, the is used : 'Smaller than the horse, the domestic donkey and African wild ass stand about 1.25 m (about 4.1 ft) high at the shoulder.[/b]
You can indeed say: A donkey is a very obstinate animal. Here you are talking in general terms. If you use ‘The donkey’, you are picking out ‘donkey’ from all other animals.
You might like to read something I’ve written for the site on articles: english-test.net/lessons/57/index.html You can then continue with (2) (3) and (4).