I found 2 in 2 different dreams (4 total) today so I'm starting to wonder if they are actually as rare as they are said to be. Also are there some that are predestined to be there like the first Gold Beanie I found was taking the place of the first enemy I was supposed to find in the first dream world since its introduction.
I've seen one in the same spot three times, myself. It isn't always there, but of the five times I have been back to that dream world, I've had three encounters (that I skipped over because I cannot be arsed try and defeat them anymore) and two regular enemies. I wouldn't say they are common, but the chances of finding one are not small.
"If you've got yourself a dream, work hard, believe in yourself because I know dreams can come true."- Dennis DeYoung 1997