Forme is changed with the weather.
-Normal forme is normal
-Sunny forme is fire
-Rainy forme is water
-Snowy forme is ice
You cant change its forme, it is dependant on what forme it was in before it evolved (as a burmy, which is affected by where it last battled).
-Trash cloak forme is bug-steel
-Sandy cloak forme is bug-ground
-Plant cloak forme is bug-grass
Forme is changed by possessing appliances.
-Normal forme is electric-ghost
-Heat forme is electric-fire
-Wash forme is electric-water
-Frost forme is electric-ice
-Fan forme is electric-flying
-Mow forme is electric-grass
Forme is changed by using the Gracidea Flower while it's daytime.
-Land forme is grass
-Sky forme is grass-flying
Any type you want it to be if you have the correct plate, and no plate for normal type.
-Using no plate makes normal
-Using meadow plate makes grass
-Using flame plate makes fire
-Using splash plate makes water
-Using sky plate makes flying
-Using insect plate makes bug
-Using toxic plate makes poison
-Using zap plate makes electric
-Using mind plate makes psychic
-Using stone plate makes rock
-Using earth plate makes ground
-Using dread plate makes dark
-Using spooky plate makes ghost
-Using iron plate makes steel
-Using fist plate makes fighting
-Using icicle plate makes ice
-Using draco plate makes dragon
-Using pixie plate makes fairy
Changes forme when a darmanitan with the ability Zen Mode has less than half the max HP.
-Normal forme is fire type
-Zen forme is fire-psychic
Changes forme by using the move Relic Song.
-Aria forme is normal-psychic
-Pirouette forme is normal-fighting
Changes forme with the prison bottle, though this has not been officially revealed and cannot be obtained.
-Confined forme is psychic-ghost
-Unbound forme is psychic-dark
Mega Charizard X: Fire/Dragon
Mega Pinsir: Bug/flying
Mega Gyarados: Water/dark
Mega Mewtwo X: Psychic/Fighting
Mega Ampharos: Electric/Dragon
Mega Aggron: Steel
Mega Sceptile: Grass/Dragon
Mega Altaria: Dragon/Fairy
Mega Lopunny: Normal/Fighting
Mega Audino: Normal/Fairy
The other source