Okay, so I have seen that cat-paw x-ray a few times, and always assumed it was fake, but before posting to say that, I did some digging, and I am super glad I did, because the truth turns out to be way more interesting!
Here's an actual paw of an adult cat:
You can easily find a lot of images like this in a simple google image search for "cat paw x-ray." (A lot of them are contrasting the normal paw with a declawed paw.) This particular one is from the imaging anatomy website at the University of Illinois School of Veterinary Medicine, which is a study tool for students to review normal anatomy.
That same Google search also shows a lot of social media posts using this image or one like it to "debunk" the ridiculously fake-looking x-ray at the top of the post. But I noticed that I wasn't seeing anything like that that was coming from something more reliable than Just Some Guy--no news articles, and no social media posts from veterinarians/students, or organizations, or well-known public science education accounts, etc.--and no one was going into any depth about it. Just random users pointing at the obvious fake and yelling "fake."
So I kept looking, and eventually found some places where the top image was posted with context, and it turned out--surprisingly! I was surprised!--not to be a fake, but an x-ray of a very young kitten, where the bones are not fully developed, and therefore don't show up well on the x-ray. This veterinary reference source has x-ray images of a neonatal puppy, including this one:
If I'm reading the article correctly, it sounds like this image--and probably the kitten one--was done at a very low power, to minimize radiation exposure to such a tiny patient, which results in a lower-quality image with less detail, contributing to the cartoony appearance.
The kitten x-ray has also been cropped to leave out the forearm bones, which--being bigger--show up with more detail in the image and therefore make the whole thing look less ridiculously fake:
(The article has a couple of paragraphs of technical detail about equipment settings and techniques to improve sensitivity and get more detail; if the person taking the kitten x-ray wasn't able to use all of those tricks, that would probably explain why the puppy one is a bit more realistic-looking.)
So there you go! Turns out teeny-tiny kittens actually are mostly fluff with cartoony little bones! Now we know.