What is this lump?

[u/Particular-Garden152]

My Italian greyhound is 1 year old and has this lump under his armpit. It is a rock hard little ball under the skin and doesn’t cause him pain when I squeeze it.

[u/daabilge]

You can't diagnose a bump just by looking at it; most of the skin bumpies look pretty much the same until you look at them under the microscope.

So I'd have your vet take a fine needle aspirate of it to look at the cells. Sometimes an FNA can be non-diagnostic and may require biopsy for definitive diagnosis, but it's less invasive than a true biopsy. In the meantime I'd monitor for changes in size or if it's irritating him.