I have seen this on a relative's dog (but relative refuses to take him to the vets). I took the dog to a free checkup in the park, and they said take him to a real vet.
I think this is an overgrowth of skin on the elbow caused by lying on laminate/tiles. It's a pressure point, so like a callus, except it's not hard skin, because it's on the elbow and possibly the elbow doesn't know how to grow hard skin - I don't know though. If either dogs were mine, I'd take them to the vet.