Sharpie Sketches - Horses by ColossalBeltloop . I once heard an artist say that she liked to draw in pen because there was no erasing, and it helped her learn to accept imperfections. That seems like a good (albeit somewhat painful ) idea!
I love these messy drawings, really appeals to my aesthetic with the messy linework and hatching. Another detail I really like is the fact that the hatching is one the same angle for all the shading, it ties the sketches together.