Ikegami to made Donkey Kong, Congo Bongo, Zaxxon and Popeye. Are they not real Nintendo and Sega games?
It's a fine line - those were contracted to ghost developers. Cotton had Success right on it, and Westone was certainly its own developer. I'm making the distinction between publisher and developer here, and while some of those blur the line (I'm sure some of the wonderboy games had sega producers involved - but what publisher doesn't do that for any game), I would say that it's harder to call Cotton a Sega game than Sonic.
My friend who is doing a wonderboy remake had to get permission from sega, yes, but he had to get a contract from westone.