This may sound dumb but i remember the first time i read The swiss family Robinson, each unknown word was described with perfection.
Your Onager reminded me of it, i am sure there is one in the story, well my version there was which would have been about 30 years ago
The point i am making is that it depends on the writer and the type of story being written.
Some writers assume that the readers are intelligent enough to deduce whilst some just like to elaborate.
If you find it hard going then go back to education or find another author!