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I would argue Seadra is the most forgettable pokemon, at least from Gen 1.
Dewgong at least had Lorelei to showcase it in-game and had several appearances in the anime.
Seadra had no major trainers use it in the games, compounded by the fact that it and Horsea were late game mons so you could play the whole game and never encounter them. And to top it off it barely appeared in the anime, even after Misty caught a Horsea.