I hit my elbow hard a few yrs ago and have a hard bump on it now that feels like bone, what could it be?

Scar tissue. Probably developed some scar tissue. Should get an xray to make sure.
Could be bone. Assuming you struck the back of your elbow, it could be a bone chip. The hook of the ulna bone is in that area, and can commonly be chipped, by striking it. If it is not a bone chip, it may be a scar nodule.