Hard to tell from the picture, but possibly a lick granuloma. Have you noticed her licking it a lot.
Not sure that they are common in pups. I've only seen them mature dogs.
Maybe you were thinking of a histoma.
That does not look like a new histoma to me.
They normally start out looking like a smooth wart like bump. Although they can look a little nasty, before they go away. That's not normally till 2 to 3 months later.
My only other thought is ringworm, but it doesn't really look like ringworm to me.
I'd have the vet look at it either way. Lick granuloma are normally treated with antibiotics, and then keeping the dog from licking it. Histomas has will go away on their own. If that's what it is.
If the vet is unsure, he would probably do a biopsy.
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