It looks mostly like a bug bite, since it's so red. The elbow callouses I've seen on many dogs have been in a different area of the elbow and usually there's one on both elbows. It does look somewhat similar to the lump Nebo had on his leg...not trying to freak you out but the vet thought Nebo's lump was cancerous (it wasn't). That's something I would definitely have checked out by a vet, just to be safe.