His rear end + thighs shrink as his back legs kick out. In addition to that, his back feet just look weird and don't seem to follow any kind of arcs at all. More importantly, however, and most fundamentally wrong, is the fact that, if this is a dog, he's hopping like a bunny.
Dogs walk like most quadrupeds. You should look up some references on animal locomotion.
With that being said, his head and ears, and front legs look pretty decent. The only technical problems outside of the aforementioned stuff I can see is that you just need to pay more attention to the changing size of your body masses over time as you animate and watch your silhouettes as they overlap between frames to be sure your movement follows some kind of arc for each and every mass attached.
You might be better off animating groups of circles or different-colored blobs of pixels to keep track of masses so that when you get your animation right on a construction level, you can add the details such as face/floppy-ears/color, etc. later.
Thankfully you approached this animation with some concern that it might need to be redone, and rightfully so. I would just suggest that you abandon any hint of detail (even the extremely simplistic style you have here) until you get the base construction of your animation 100% correct to save yourself a LOT of mini-hassles from being distracted w/maintaining the shape of the head / mouth / eyes / flop of the ears, etc. etc. -- it will make your workflow go SO much easier.
