I have a cow that has what looks like an abscess on the bottom of her brisket, almost between her front legs. It is about the size of a cantaloupe. Will it eventually go away on its own or will the vet need to lance it?
Where are you located? Any chance you are at a high elevation? (brisket disease) https://www.beefmagazine.com/health/0801-brisket-disease-cattle
Are you sure it is new? Maybe just a very fat cow with a heavy brisket?
The brisket being the lowest point is where most of the oedema gravitates to so while it is possible that an abscess is there look around elsewhere further up for an injury like a haematoma.