just a quick one because not a whole lot is off. I left the tail alone since it was intentional; it looks fine in that case c:
The top is things I think you should change, and the bottom is things that you could go either way on. As in how I would personally do it.
So first off, if we go from left to right, I noticed your nose looked a little off. They're sort of mushroom shaped, but there shouldn't be a big opening between the sides (you could start with a circle and add nostrils, or just draw a wonky mushroom like I usually do).
Shortening the neck was the biggest thing; when the neck gets too long it just looks giraffish, and it also puts the pose out of balance. I'd suggest a bit more broadening as you go from the neck into the chest too, vs a more consistent width.
And then just lifting up the shoulder makes the pose looks less static and more mobile. The very dropped shoulder looks like it's a bit bent out of place, and holding a lot more weight than the dog is able to put on it. The leg itself is perfect though.
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On the bottom one, I just redrew the head the way I would do it. I would suggest narrowing your muzzle a bit more and rounding out the head a bit, simply because a slender, pointed snout is a border collie feature, and overemphasizing it will give a better breed recognition than underdoing it will.
And I also redrew the back leg, because even though I can't find anything wrong anatomically, it still looks a little awkward to me. Having it stretched back makes the pose look more balanced to me, but I think that's opinion more than anything, so your call.
If you stick with the longer tail, you could also have it out straighter in a more cat-like manner; it might add some intensity to the pose. Just a thought, not even really a suggestion XD
But yeah, you did a really good job on the legs and body, just some little things about the face and neck.