I’ve heard about these guys too. Never one attached to a fish, but I’ve seen the standard lamprey wound, mostly on rough fish that have washed up dead on the banks.
I did a couple of days of volunteer stream survey work with the WI DNR a few summers ago. I was shocked to see the number of lampreys that turned up in what most would consider to be a premier trout river in Western WI. The field biologist we were working with said that lampreys are an indication of excellent water quality (temp, chemistry, chow, etc). Still a little creepy to see those little buggers come writhing out of the weeds towards your stream shocker probe!