No, I do not believe Castform is a legendary. I think he was made by Humans in order to control the weather.
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no no it just changes form
No, castform morphs due to the weather.
yes:)
Castform doesn't evolve it can however change forms during rain dance, sunny day and hail.
Castforms forcast ability will enable it to change its form and type depending on what the wheather is like. If it is raining castform will change into its water form, if sunny castform will change into its fire form and if it hails it will transform into its ice form.