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"Baby" Pokemon is a game mechanic. You need to lose the idea that "Baby" Pokemon means ALL immature Pokemon, or everything along those lines. Essentially it's a mechanic introduced in gen 2 as another way to "catch 'em all" - originally it was simply Pokeballs, gifting or trading. Then they added breeding, alternate evolutions, and form changes. These are all simply mechanics to make the capturing aspect of the games expansive, unique and not easy.
I mean if you really want to get into it, Happiny isn't even the same species as Chansey or Blissey; it simply evolves into those species. Pokemon very rarely uses such terminology to mean something other than a particular game mechanic.