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!
now this is really impressive, it's a gesture drawing in 30 seconds, and you can just see the professionalism in these lines. this person knows what they're doing and you can get to their level only with practice