Requires Entity Model Features or Optifine (not tested but seemingly should work) mod for custom vanilla mob models.
I'm very surprised no one did a resource pack like that (or I really missed it) before, despite seeing a lot of various fox model packs!
This is my first experience with Blockbench (I only changed the model, adjusting the texture outside it)
Since foxes were added back in 1.14, I was wondering: why their heads are so wide? In my opinion it actually has two issues.
First one is its remarkable facial expression, which looks a bit "dumb" with these strange 3 pixel wide eyes. And it looks quite different from the style of existing animals, that atleast to a degree follow general proportions of their real counterparts, and about that...
Second one is how badly in my opinion it represents not just proportions but even the "idea" of a real fox appearance, which meant to be sleek and elegant, with its face being that of an agile and sly hunter! Having a head that, for some reason, is wider than the body, with such "detached" eye look, quite barely fits the real animal and its "idea".
After all, wolfs have more or less realistic model!

So, I made this simple resourcepack that changes the fox model in the way it should had been implemented in the first place, in my opinion, looking both more like real fox, and more "Minecrafty". I compressed width of the head from 8 to 6 pixels, and its height from 6 to 5. I compressed the snout from 4 pixels wide and 3 pixels long to 2 and 2, making it 2x2x2 cube, giving it much more sharp and "canine" look. Also, I lowered the height of the tail from 5 to 4 pixels, making it the same as its width. It still looks fluffy enough! Then I adjusted the texture. I had a temptation of changing it more, making it more like real fox, but I decided to keep the vanilla texture (which actually looks quite old style) instead, only adjusting it to the new model. And of course I gave it the classic Minecraft eyes!
The only issue is that item in mouth holding position remains on the old spot (I almost lost my mind trying to change it), so item is one pixel below the head. However, you would notice that only on close up distance with fox looking at you, and in survival it rarely happens, with foxes usually running away from you.

It should work on other versions as well, if model or EFM properties weren't changed. You can freely adjust it to your version.

Normal Fox