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Now that Metagross was mentioned.. I think one reason why Fairy type was made, and Steel type's resistances to Ghost and Dark types was removed..
...perhaps it was to nerf the Pseudo-Legendaries?
A Normal/Fairy Blissey:
Immune to Ghost and Dragon
Takes neutral damage from Fighting.
Weak to Poison and Steel.
Resists Dark and Bug.
Has high SpDef.
That's it I'm getting a Chansey, again.
Could be that the Helix are referring to Mega Evolutions.
Edit: It does refer to Mega Evolutions. I can see that icon on the HP bar on every Mega Evolved Pokémon.