Update 2/9: Castform’s form depends on the weather meaning that if you catch it when Cloudy and Partly Cloudy it will be in its Normal Form, when Sunny/Clear, Windy and even at night – Sunny Form, Rainy – Rainy Form, Snowy – SNowy Form. Have in mind that Castform and its moves will not change, it will retain that form and moveset.
Previous story: The Pokemon Go’s Castform, or known better by the name “Weather Pokemon,” has the ability to shapeshift depending on the current weather. As we know, this “Weather Pokemon” has a max CP of 1486. The stats are balanced but are on the low side. The ATK (139) is not that strong for a battle, and the defensive stats won’t make Castform last in a battle too.
This Pokemon has 4 forms. During a normal weather, Castform is in its Normal form and it is a normal type. During the blazing sunlight, we can see it in its Sunny form, which means that it is a fire type. During a rainy weather, it is in its Rainy form, and it is a water type. When it is raining snow, logically, it is in its Snowy form and it is an ice type.
We know that only its Normal form/type has a shiny variation. The other three forms do not have any obvious changes. To detect whether you have a shiny or a normal Castform, is the difference in its color. A normal Castform is gray with a lock which is extending from the top of its head. It has white eyes and black pupils. The shiny Castform is light pink with yellow eyes and black pupils.
It is not a Pokemon that will have a place in the tier/rank lists. It is just a Pokemon for Pokedex collection. Right now, our only hope is that they release it together with the Generation 3 Pokemon. Also, Castform does not have any further evolution.