Lump

I was wondering what this lump is.

Hello.  I would prefer if it this lump turned out to be one of those realistic-looking fake lesions that can be made up on the skin nowadays, but assuming that this is a real skin lump on a real dog, it may turn out to be a real cause for concern.  Hopefully your vet has seen it: they may be prepared to either remove or biopsy it.  If it is real, then complete removal may be difficult and may even involve leg amputation to perform, which is why some vets may prefer to biopsy or take x-ray pictures to understand the lesion first. Either way, pain releif and antibiotic  cover may be extremely important right now, so hopefully a vet has seen this lump or will do so as soon as possible.  With regards to possibly identities, there are many possibilities including various cancers, cysts, abnormal hygromas, infectious lesions, infected wounds, so I will leave the diagnosis to your vet.  Wishing your dog the very best of luck going forward - please will you keep us updated?