i discovered a surefire method of forging heavy ones, especially 14, 15lb. ones. just let all the slots in your only 100% WR bag fill up, and sure as anything you'll forage up the heavy ones in non WR slots.
This is NOT a drop from creatures... at least after killing hundreds of them. I did forage them (as stated elsewhere in diff. lbs.) with my bard while fishing in Natimbi...