In what instances do Pokemon lose or gain types?

Question

I know we have Forest's Curse (adds Grass-type), Trick-or-Treat (adds Ghost-type), Burn Up (removes Fire-type), and some evolutions (including Mega-Evolutions) [Mega Aggron for example]. But, is there anything else that adds or removes types on Pokemon?

Answer 1

I only included moves, Abilities, and form changes/Mega Evolutions in this answer. Evolutions don't count, in my mind, but if you want me to include them just tell me and I'll try and find a list somewhere. Also, please tell me if I missed something!

Forest's Curse: Adds Grass-type to the target.
Trick-or-Treat: Adds Ghost-type to the target.
Soak: The target loses their typing and gains the Water-type.
Burn Up: Removes Fire-type from the user.
Roost: Removes Flying-type from the user for one turn.
Conversion: Changes the user's type to the type of the move in its first moveslot.
Conversion2: Changes the user's type to a type that resists or is immune to the target's previous attack.
Colour Change: User loses previous type and gains the type of the move used by the opponent.
Camouflage: User loses previous type and gains the type of the environment.
Protean: User loses previous type and gains the type of the move it's about to use.

Mega Charizard X loses the Flying-type and gains the Dragon-type.
Mega Pinsir gains the Flying-type.
Mega Gyarados loses the Flying-type and gains the Dark-type.
Mega Mewtwo X gains the Fighting-type.
Mega Aggron loses the Rock-type.
Mega Sceptile gains the Dragon-type.
Mega Altaria loses the Flying-type and gains the Fairy-type.
Pimal Groudon gains the Fire-type.
Mega Lopunny gains the Fighting-type.
Mega Audino gains the Fairy-type.

Castform loses the Normal-type and gains the Fire-type in Harsh Sunlight, the Water-type in Rain, or the Ice-type in Hail.
Rotom loses the Ghost-type and gains the Water-type in Wash form, the Fire-type in Heat form, the Flying type in Fan form, the Ice-type in Frost form and the Grass-type in Mow form.
Shaymin gains the Flying-type in Sky Forme.
Arceus loses the Normal-type and gains the type of the plate it holds.
Darmanitan gains the Psychic-type in Zen Mode.
Meloetta loses the Psychic-type and gains the Fighting-type in Pirouette form, when it uses Relic Song.
Hoopa loses the Ghost-type and gains the Dark-type in Unbound form.
Oricorio loses its primary type and gains the Fire-type, Electric-type, Ghost-type or Psychic-type depending on what nectar is used.
Silvally loses the Normal-type and gains the type of the Memory disc that it holds.

Comments

Actually, Converson changes the user's type to match the type of one of its own moves.  In Gens 1-6, it would randomly choose from the user's moveset, but in Gen 7, it will now change to match the type of the user's first moveslot.

Source: https://bulbapedia.bulbagarden.net/wiki/Conversion_(move)

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Ah, thank you!

Answer 2

Here are all the things you did not mention.

Moves:

Roost:
If you are a flying type, you will no longer be one after using it.

Reflect Type:
Becomes the same type as your opponent.

Soak:
Changes opponents type into a water type.

Camouflage:
Changes your type according to the environment.

Conversion:
Randomly changes your type to one of your moves.

Conversion 2:
You become a type that is "Resistant" to the attack you last received.
Ex. You get hit by a flying type move, you will change into a type that flying is not effective against like Rock or Steel.

Abilities:

Multi-type and RKS System:
Changes your type depending on the item(plates). Only Arceus and Silvally know it.
Ex. If you have a Flame plate, you will become a fire type.
I think that's all. I hope this helps!

Comments

Thanks. I forgot those.

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Oh, Reflect Type! I knew I forgot something.