I used to draw a LOT of horses.
For about four years, I drew practically nothing but horses, dogs, and dragons. Understandably I got good at drawing horses, dogs, and dragons. But when I reached the end of that stage and developed an interest in drawing people, I suddenly hit a wall. Though I'd developed a great anatomical understanding of animals, little of it applied to people. (Look, I think that people are animals. But there's a big difference in musculature and movement between a digitigrade quadruped and a plantigrade biped.) It was like I'd lost about a year's worth of gains in drawing skill just because I switched subjects.
That was a bit depressing, as you might imagine. But I kept drawing people and anything else that I was in the mood to draw, and I got better at it. As I got a bit older and a bit more varied in my drawing diet, horses gradually dropped out of my regular drawing rotation.
Ten years on, and now I'm on the other side of the same wall. I'm now much better at drawing people than I am at drawing horses, and it's a bit depressing. When I sat down to draw this horse, it was sort of like my hand contracted a sudden case of terminally stupid.
Le sigh. Tortured artist is tortured, I suppose.