Q. I have a 11 year old boxer / mastiff cross, yesterday I noticed a lump the size of a orange on his left side under his skin over his rib cage.
A. Oh dear, these types of things can make any pet parent fret. Unfortunately, boxers can be prone to certain types of cancers but that does not mean that this is cancer. Such a large lump is cause for concern, as it could be a fairly large injury or fat lump which could be causing discomfort or something more serious. I would book a vet appointment so they can take a biopsy and see why it formed. Hope its nothing serious.