Here is the list of Pokèmon that gets type-modification after evolution
Note: I excluded mega evolutions because they are temporary and It cannot be considered as an evolution but a form change.
Layout : Evolved Pokèmon{form}(Type attained by evolution)(type1 gets replaced by type2)
- Charizard (Flying)
- Butterfree (Flying)
- Alolan Raichu (Psychic)
- Nidoqueen (Ground)
- Nidoking (Ground)
- Alolan Ninetales (Fairy)
- Bellossom(Poison removed)(Grass type)
- Poliwrath(Fighting)
- Galarian Rapidash (Fairy)
- Galarian Slowbro (Poison)
- Dewgong (Ice)
- Cloyster (Ice)
- Steelix (Rock gets replaced by Steel)
- Alolan Exeggutor (Psychic gets replaced by Dragon)
- Alolan Marowak (Fire/Ghost)
- Galarian Weezing ( Fairy)
- Kingdra(Dragon)
- Starmie(Psychic)
- Galarian Mr. Mime (Fairy gets replaced by Ice)
- Scizor (Flying gets replaced by Steel)
- Gyarados (Flying)
- All Eeveelutions (Normal gets replaced by any one among Water,
Electric, Fire, Psychic, Dark, Grass, Ice, Fairy)
- Dragonite (Flying)
- Togetic (Flying)
- Marill(Normal gets replaced by Water)
- Forretress(Steel)
- Magcargo (Rock)
- Tyarintar (Ground gets replaced by Dark)
- Combusken (Fighting)
- Marshtomp (Ground)
- Beautifly (Flying)
- Dustox(Poison)
- Nuzleaf (Dark)
- Gallade (Fairy gets replaced by Fighting)
- Masquerain (Water gets replaced by Flying)
- Breloom(Fighting)
- Ninjask (Ground gets replaced by Flying)
- Shedinja (Ground gets replaced by Ghost)
- Probopass (Steel)
- Vibrava (Dragon)
- Cacturne (Dark)
- Altaria (Normal gets replaced by Dragon)
- Crawdaunt (Dark)
- Froslass (Ghost)
- Salamence (Flying)
- Torterra (Ground)
- Monferno (Fighting)
- Empoleon(Steel)
- Bibarel (Water)
- Mothim(Flying)
- Wormadam{Plant cloak}(Grass)
- Wormadam{Sandy cloak}(Ground)
- Wormadam{Trash cloak}(Steel)
- Gastrodon (Ground)
- Lucario(Steel)
- Drapion(Bug gets replaced by Dark)
- Pignite(Fighting)
- Excadrill(Steel)
- Palpitoad (Ground)
- Escavailer (Steel)
- Chesnaught (Fighting)
- Delphox (Psychic)
- Greninja (Dark)
- Diggersby (Ground)
- Fletchinder (Normal gets replaced by Fire)
- Vivillon(Flying)
- Pangoro(Dark)
- Dragalge(Water gets replaced by Dragon)
- Decidueye(Flying gets replaced by Ghost)
- Incineroar(Dark)
- Primarina(Fairy)
- Charjabug(Electric)
- Crabominable(Ice)
- Hakamo-o(Fighting)
- Solgaleo (Steel)
- Lunala (Ghost)
- Nagandel (Dragon)
- Corviknight (Steel)
- Dottler (Psychic)
- Drednaw (Rock)
- Carkol (Fire)
- Hatterene (Fairy)
- Urshifu{Single Strike Style}(Dark)
- Urshifu{Rapid Strike Style}(Water)
- Galarian Slowking (Poison)
- Wyrdeer (Psychic)
- Kleavor (Rock replaces Flying)
- Ursaluna (Ground)
- Basculegion (Ghost)
- Meowscarada (Dark)
- Skeledirge (Ghost)
- Quaquaval (Fighting)
- Lokix (Dark)
- Pawmo (Fighting)
- Armarouge (Psychic)
- Ceruledge (Ghost)
- Scovillain (Fire)
- Rabsca (Psychic)
- Annihilape (Ghost)
- Gholdengo (Steel)
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Thank you!
Source: My experience and this site!