I sketched a rhinoceros based on the image in the book The Weatherly Guide to Drawing Animals by Joe Weatherly.
Next I decided to sketch a closeup, on my own: no video, no book, no instructions. This isn’t as detailed as I wanted it to be. It was hard. Actually, they are all hard for me and take a lot of effort. I look forward to drawing this again, when I have more experience.
It's a long story ... 
Love your effort…and the drawing…and…the admission that it is hard…Keeep up the good work.
That’s amazing without a reference. Looking forward to more sketches.