For starters, this is a very well-rendered run cycle. You did a great job with the lineart and colouring for each frame in the cycle. The one and only issue I have with the rendering appears to be one frame where the body colour is slightly darker than the rest of the frames.

You did a great job with many of the "minor details" which people sometimes neglect in run cycles, especially the movement of the mouth and showing how the underlying form changes throughout the stride (for example, in the torso.) I think you could have done more with the movement of the ears, but that depends on how stiffly they are held - still, I think there would be a more noticable movement of the ears. Also, the tail changes length quite noticeably throughout the cycle - I think you could have done a bit more with the movement of the tail as well to emphasize/reflect the running flow of the entire form.

There seems to be an overall flaw in your run cycle, and that is that you haven't captured the essence of running (I'll explain what I mean by that, bear with me!) [link] is a beautiful video to illustrate what I mean - the act of running is an act of not simply moving the legs but pushing the body forward across the ground. When your character's hind legs connect with the ground, they seem to simply slide across a frictionless surface rather than connecting with solid ground and exerting the force required to propel the character's weight forward. Observe the timing of the lioness's running and the way her muscles bunch when her legs connect with the ground. When she pushes off from the ground, it's an almost spring-like motion. The hind legs seem to move more quickly when they are contact with the ground (in other words, when that spring-like force in her legs is pushing her forward) than when she is bringing them forward again.

I'd like to also address the movement of the front legs. The front legs seem stiff - unlike the torso which bunches and lengthens in your character, the muscles of the front legs seem to be locked in place. In the video I linked, though, you'll see that the lioness's front legs ripple as the muscles continually flex. When she lifts her front legs from the ground, she brings the legs closer to her chest. The elbows curl back and her paws are tucked in before she reaches to "grab" more ground. When she does reach her front legs forward, the entire body seems to angle downward - her head is lowered and her front legs are lowered as they make contact with the ground.

Overall, with regards to the front legs, this character doesn't seem to be taking his body weight or moving in a fluid, realistic manner. You did an excellent job with the movement of the rump and spine, but then you seem to lose that motion in the shoulders - if you watch the lioness running, it almost looks like a ripple which travels from her rump, up her spine, and becomes a sort of forward thrust in the shoulders as she makes a leap forward to "grab" more ground.

The movement of the shoulders is essential here. The shoulders of this character appear to remain stationary, but the shoulders should flow with movement. The lioness's shoulderblades sink when her legs contact the ground and rise when she lifts her front legs to grab more ground.

What I should emphasize in closing, though, is that this is an amazing run cycle regardless of the issues I have pointed out. The frames are extremely consistent (except for that tail, hehe) and overall it's helped by your solid understanding of surface anatomy and proportion. I also forgot to take into account the fact that you're missing a frame - I think most of what I said still applies, but I apologize if my observations/criticisms might be mis-applied because of that missing frame. Still, I hope you find this critique insightful and helpful