The legs just look like they're moving back and forth.
You need more frames with the paws looking like they're on the ground, struggling to produce the effort to push it's weight back up and forward.
The head is moving around in really jerky ways, meaning it's moving too fast and exposing your frame rate. It's like with pixel art, you want to make it look as close to possible to infinite resolution, with animation you want to achieve infinite FPS, or at least the illusion of it.
Keep your jerky movements for use of emphasis, like when the feet push off.
The movement looks natural (to me), so nice job on that. (I don't own a cat or anything, so it's hard for me to say. Make some bird art and I can help more in that respect.
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On the art in general: The contrast is low, and the cat looks a little bland.
Nice job on that anatomy, but the head looks sort of wrong...
