This is Storm today. She looks pretty happy. That's because her toe is not hurting nearly so badly as it was 10 days ago. Now she can walk on her right hind paw, though it still gets sore toward the end of the day.
Here are normal toes on the left, and the affected toe on the right. The third phalanx (last bone at the tip, also abbreviated P3) is the bone the nail grows from. Notice that the P3 on that bad toe just looks like it exploded. (Sorry my pictures are not as well in focus as I'd like.)
That could be anything from a bone infection (osteomyelitis) to bone cancer. There could have been a wound, a nail pulled loose and infected, a penetrating foreign object. A biopsy could be your next step (taking a small piece of abnormal tissue). Sometimes that doesn't give you all the information you'd like to have. In this case, the clients opted for a trial therapy of antibiotics first.
If you don't have bone cancer (or some other kind of cancer that happens to be eating into the bone nearby) and if you picked the right antibiotics (and they get into the bone), it might get better. If it does, great, and keep taking your medicine for the next four to six weeks. If not, it's biopsy time, or maybe even amputation time (the toe, not the leg).
And it is better, at least 50% better. The swelling is down by half, and (as noted above) she can walk most of the day on it before it starts getting sore. So, we're optimistic. It may still "go south" on us, but we're optimistic (and continuing treatment).