Does anyone know what this lump is on this deer's spine?

Some animals get growths on them. Could possibly be where a bullet or arrow or car knicked him at some point. COuld be a solid growth, could be filled with puss. Might kill it, might not hurt anything.
-Jake

Cutaneous fibroma. Basically a wart. Never seen them on deer around my place, but watched a herd of mulies in Yellowstone with them. Kinda gross looking. Google it.

It does look like a fibroma. I've only seen one deer with those. They don't harm the deer and the venison is safe to eat from what I've read .

Could be a bot fly. Lay a larvie under there skin but that looks big. We get them on our cattle if we don't vacinate but I live in the west. Maybe you don't get them in your neck of the woods.