Anatomy. This is what I am interested that you understand. If you want to learn to draw a realistic animal, you should study anatomy!
The correct position of the bones determined the basis for body volume, the correct location of the limbs, etc. You should understand what you are drawing! So, if you want to draw a realistic penguin , please do not stay just with this basic tutorial and go to find pictures on the internet or in books and STUDY them!
Now if you go to a zoo to see live animals .. even better!