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Volcarona was supposed to be a legendary pokemon. It's an overworld encounter at the end of a dungeon, has a place in Unova's history, and it has abnormally high stats.
The reason it was changed was to cut down on the number of legendary pokemon. They didn't want to exceed the number introduced in Gen 4 since people complained it made them feel not as special.
So they reduced its stats, quickly made a pre evolution for it, and gave it out as a random egg on a lategame route.
