Here's one thing that's always irritated me in fiction. Lots of people look at the posterior-pointing joint on the rear legs of many vertebrate animals (with the exception of fish and snakes) and think that it's a knee, except reversed.
The result? Whenever humans transform into animals, there's often a description of the knees "reversing" on themselves, forcing the human onto their fours.
With their legs like that, how do they run, I wonder? You need a reliable way to lift and push your leg off and against the ground, and the best way to do that is with a knee.
Take another look at this wolf. See here that there are two joints in the rear limb: a knee and then an ankle. Wolves, and most other quadruped animals, walk on the tips of their toes.
'Kay, technically the ankle is called a hock, but it is homologous the ankle, and isn't that far off in its structure, either. I call it the ankle in the same way I sometimes call the forelimbs "arms".
The real knee? It's far more medial. Most people miss it because the thigh is so short and tucked into the abdomen, so the animal's in a permanent squatting position. The reason why squatting is much more natural for them because they have short thighs and long feet, and their toes are better-suited for supporting weight.
So, in the case of an animal transformation? It's far more probable that the bones of the leg will lengthen and shrink rather than the knees actually reversing, turning into hocks, the tarsals and tibiae/fibulae merging, the femurs splitting into tibiae/fibulae, and two new femurs springing from the hip joints out of nowhere.
And that's just the skeleton. Think of how all the muscles in those regions would have to split or merge to convert themselves into other muscles, and how new muscles for the thigh will also have to spring out of the hip joints from nowhere along with the femurs.
That's a lot of unnecessary work for a shapeshifter to do!
Whenever you describe or draw an animal transformation, it only makes sense that homologous structure matches to homologous structure. Arms turn into wings, be they of a bat or a bird. The coccyx of a human becomes a tail. A part of the human gullet expands to become a bird's crop.
God help you if you're trying to transform into an invertebrate, however, as there are no homologous structures, except maybe in embryonic form. Good luck!
