Season trio are created by Arceus's most loyal Pokemon,Neucentaur after Groudon and Kyogre fell into slumber. Neucentaur itself is part of Season Trio,representing Spring and Autumn. He created Hinatsuryu,The Summer Pokemon and Coldsibidra, The Winter Pokemon. They created a cycle we know as seasons. Then they stabilized The Pokemon World and Neucentaur is blessed by Arceus. Neucentaur is blessed with ability Season Change that could make him use the powers of another members of Trio with using moves called Extreme Sunshine and Extreme Blizzard. And this is their type.
Neucentaur : Steel/Dragon type
Hinatsuryu : Fire/Dragon type
Coldsibidra : Ice/Dragon type
Coldsibidra's ability is Extreme Winter,which will add +33% damage for Ice type move and -33% damage for any other move except for Dragon.
While Hinatsuryu's ability is Extreme Summer,which will add +33% damage for Fire type move and -33% damage for any other type except for Dragon.
The ability of Neucentaur is Season Change and Neutralization. Neutralization will remove the effect of Extreme Winter and Extreme Summer.
If Neucentaur became Summer Form or Winter Form (because of Season Change) Neutralization will be replaced with Extreme Summer or Extreme Winter.