A lipoma is just a lump of fat, a proliferation of fat cells. They are relatively common among older dogs. In cats, they are less common. They can occur in all parts of the body under the skin and usually grow slowly. In general, these lumps are entirely harmless and can remain in place with peace of mind. Only if the fat hump is in an annoying place, causing the animal to suffer (for example, in the armpit) can it still be decided to remove the hump. Usually, the lumps will stay away, but there is still a risk that new lumps will appear in the same place or another place.