All Pokémons evolved before evolution level:
(Excuse me! I wouldn't include Rocket grunts or scientists because they tons of underlevelled 'mons)
Red & Blue:
- Route 11 - Engineer (Bernie) - has a lvl 18 Magneton (evolves at lvl 30)
- Rock Tunnel - Hiker (Lucas) - has a lvl 21 Graveler (evolves at lvl 25)
- Route 8 - Super Nerd (Glenn) - has a lvl 22 Muk (evolves at lvl 38)
- Route 8 - Gambler - has a lvl 22 Poliwhirl (evolves at lvl 25)
- Pokemon Tower 5F - 2 Channelers - have lvl 22 and 23 Haunters (evolves at lvl 25)
- Saffron Gym - Psychic (Cameron) - has a lvl 33 Slowbro (evolves at lvl 37)
- Route 16 - Biker (Hideo) - has a lvl 33 Weezing (evolves at lvl 35)
- Route 17 - Biker (Ruben) - has two lvl 28 Weezings
- Route 17 - Biker (Billy) - has a lvl 33 Muk
- Route 17 - Cue Ball (Jamal)- has a lvl 26 Machoke (evolves at lvl 28)
- Route 17 - Biker (Jaxon) - has lvl 29 Weezing and Muk
- Route 17 - Biker (Williams) has 2 lvl 29 Weezings (25 in FrLg)
- Route 14 - Biker (Gerald) - has a lvl 29 Muk
- Route 15 - Biker (Ernest)- has a lvl 29 Weezing (25 in FR, LG)
- Route 15 - Bird Keeper (Chester)- has a lvl 28 Dodrio (It evolves at lvl 31)
Note: The names in bracket are in FrLg
Yellow:
- Route 6 - Jr. Trainer - has a lvl 16 Raticate.
The rest same as Red and Blue.
Gold & Silver:
- Violet Gym - Faulkner - has a lvl 9 Pidgeotto (evolves at lvl 18)
- Union Cave - Hiker Phillip - has a lvl 23 Graveller
- Ecruteak Gym - Medium Grace, Medium Martha - have lvl 20 Haunters + Other Haunters
- Route 38 - Beauty Valerie - has lvl 17 Skiploom (It evolves at lvl 18) (not in HGSS)
- Route 39 - Sailer Eugene - has lvl 17 Poliwhirl and Raticate
- Glitter Lighthouse - Sailor Huey - has a lvl 18 Poliwhirl + Other Poliwhirls
- Route 41 - Sailor Charlie - has a lvl 19 Tentacruel (evolves at lvl 30)
- Route 41 - Swimer Kailee - has a lvl 20 Seaking (evolves at lvl 33)
- Route 43 - Camper Spencer - has a lvl 17 Sandslash (evolves at lvl 22)
- Route 43 - Fisher Marvin - has lvl 10 and 15 Gyarados (evolves at lvl 20) (Not in HGSS)
- Route 43 - PokéManiac Ben - has lvl 19 Slowbro (Not in HGSS)
- Mahogany Gym - Boarder Ronald - has a lvl 25 Dewgong (evolves at lvl 34)
(Other Dewgongs in Mahogany Gym)
- Mahogany Gym - Pryce - has a lvl 31 Piloswine (evolves at lvl 33)
- Route 26 - Cooltrainer Beth - has a lvl 36 Rapidash (evolves at lvl 40) (not in HGSS)
- Indigo Plateau - Lance - has 3 Dragonites, 2 of lvl 47, 1 of lvl 50 (evolves at lvl 55)
- Saffron Gym - Medium Doris - has a lvl 36 Slowbro
- Route 8 - Super Nerd Sam - has a lvl 34 Muk
Ruby & Sapphire:
- Mauville Gym - Wattson - has a lvl 23 Magneton
- Route 114 - Sr. and Jr. Tyra & Ivy - have a lvl 21 Graveler
- Mt. Chimney - Maxie (Ruby) - has a lvl 25 Camerupt (evolves at lvl 33)
- Mt. Chimney - Archie (Sapphire) - has a lvl 25 Sharpedo (evolves at lvl 30)
- Petalburg Gym - Norman - has two Slakings of lvl 28 and 31 (evolves at lvl 36)
- Fortree Gym - Winona - has a lvl 33 Altaria (evolves at lvl 35)
- Route 121 - Sr. and Jr. Kate & Joy - have a lvl 32 Slaking
- Route 123 - Cool Trainer Clyde - has a lvl 29 Magneton
- Magma Hideout (Ruby) - Magma Admin Tabitha - has a lvl 32 Camerupt
Note: Included Magma Hideout because it would be too less without them. And I would add the Villian's Pokemons further.
Emerald:
- Mauville Gym - Wattson - has a lvl 24 Manectric (evolves at lvl 26)
- Lavaridge Gym - Flannery - has a lvl 26 Camerupt
- Desert - Ruin Maniac Byran - has a lvl 22 Sandslash
- Route 115 - Triathlete Kyra - has a lvl 26 Dodrio
- Route 108 - Cool Trainer Carolina - has two lvl 24 Manectrics
- Route 108 - Swimmer Jerome - has a lvl 25 Tentacruel (evolves at lvl 30)
- Route 109 - Fisherman Carter - has a lvl 25 Tentacruel
- Route 123 - Cooltrainer Braxton - has a lvl 28 Magneton
The rest are the same from RS except Clyde and Norman (has lvl 31 slaking only).
FireRed & LeafGreen:
- Route 11 - Engineer Braxton - has a lvl 21 Magneton
- Route 11 - Youngster Yasu - has a lvl 17 Raticate
- Rock Tunnel (B1F) - Hiker Dudley - has a lvl 21 Graveler
- Route 10 - HikerTrent - has a lvl 19 Graveler.
- Route 8 - Biker Ricardo - has a lvl 23 Grimer
- Route 8 - Gamer Stan - has a lvl 22 Polihirl
- Pokemon Tower 5F - Channeler Tammy - has a lvl 22 Haunter
- Young Couple Lea & Jed - has a lvl 29 Rapidash
- Route 14 - Biker Lukas - has a lvl 26 Grimer
- Route 15 - Biker Alex - has a lvl 28 Weezing
- Route 19/20 - Swimmer Axle - has a lvl 27 Tentacruel
- Route 19/20 - Swimmer Alice & Anya - have two Seakings of lvl 30 and 27
Diamond & Pearl:
- Valley Windworks - Commander Mars - has a lvl 16 Purugly (evolves at lvl 38)
- Team Galatic Eterna Building - Commander Jupiter - has a lvl 20 Skuntank (evolves at lvl 34)
- Veilstone Gym - Black Belt Darren - has two lvl 25 Machokes
(Other Machokes in the gym)
- Route 214 - Psychic Mitchell - has a lvl 23 Haunter.
- Fuego Ironworks - Worker Conrad - has a lvl 33 Rapidash
Platinum:
- Valley Windworks - Commander Mars - has a lvl 17 Purugly (evolves at lvl 38)
- Team Galatic Eterna Building - Commander Jupiter - has a lvl 23 Skuntank (evolves at lvl 34)
- Hearthome Gym - Ace Trainer Allen - has a lvl 24 Haunter
(other Haunters in the Gym)
- Route 210 - Belle & Pa Ava & Matt - have a lvl 24 Rapidash
- Route 215 - Ace Trainer Dennis - has a lvl 24 Drifblim (evolves at lvl 28)
- Pastoria Gym - Fisherman Erick - has a lvl 31 Seaking
- Sevenstars Restaurant - Socialite Rebecca - has a lvl 27 Purugly
- Route 216 - Ace Trainer Maria - has a lvl 39 Rapidash
HeartGold & SoulSilver:
- Route 35 - Bird Keeper Byran - has a lvl 14 Pidgeotto
- Olivine City - Beauty Charlotte - has a lvl 16 Bellossom (Gloom evolves at lvl 21)
- Cianwood City - Eusine - has a lvl 27 Electrode (evolves at lvl 30)
- Route 42 - Hiker Benjamin - has a lvl 17 Dugtrio
- Route 43 - PokéManiac Beckettt - has lvl 20 Slowbro
- **Mahogany Town (Rocket HQ) - Lance (ally) - has a lvl 40 Dragonite
- Route 26 - Ace Trainer Jamie - has a lvl 36 Rapidash
Black & White:
- Castelia Gym - Burgh - has a lvl 21 Whirlipede (evolves at lvl 22) (not in B2W2)
- Route 7 - Youngster Parker - has a lvl 29 Scolipede (evolves at lvl 30)
- P2 Laboratory - Scientist Nathan - has a lvl 35 Garbodor (evolves at 36)
- Opelucid Gym - Drayden / Iris - have lvl 43 Haxorus (evolves at lvl 48)
- Team Plasma Ghetsis - has a lvl 54 Hydreigon (evolves at 64)
Black 2 & White 2:
- Virbank Gym - Roxie - has a lvl 17-19 Whirlipede
- Driftveil Gym - Clay - has a lvl 28 Krokorok (evolves at lvl 29)
- Team Plasma Ghetsis - has a lvl 52 Hydreigon
- Pokémon League - Champion Iris - has a lvl 56 Hydreigon
X & Y:
- Route 5 - Rising Star Hamish - has a lvl 13 Kadabra (evolves at lvl 16)
- Battle Chateau - Countess Adele - has a lvl 25 Shelgon (evolves at lvl 30)
- Battle Chateau - Countess Eliane - has a lvl 25 Lairon (evolves at lvl 32)
- Battle Chateau - Countess Patricia - has a lvl 25 Doublade (evolves at lvl 35)
- Battle Chateau - Earl Sancy - has a lvl 25 Dragonair (evolves at lvl 30)
- Battle Chateau - Earl Lesotho - has a lvl 25 Fraxure (evolves at lvl 38)
- Battle Chateau - Marchioness Amandine - has a lvl 35 Avalugg (evolves at lvl 37)
- Battle Chateau - Marchioness Amelie - has a lvl 35 Wailord (evolves at lvl 40)
- Pokéball Factory - Two Team Flare Grunts - have Scraggy of lvl 36-37 (evolves at lvl 39)
OmegaRuby & AlphaSapphire:
- Route 109 - Ace Trainer Portia - has a lvl 28 Magcargo (evolves at lvl 38)
- Mauville Gym - Wattson - has a lvl 21 Magneton
- Mt. Chimney - Maxie (OR) - has a lvl 27 Camerupt + other Camerupts
- Mt. Chimney - Archie (AS) - has a lvl 27 Sharpedo + other Sharpedos
- Petalburg Gym - Norman - has two Slakings of lvl 28 and 30
- Secrat Base Guild - Aarune - has a lvl 23 Flygon (evolves at lvl 45)
Sun & Moon:
- Almost all Totems
- Ten Carat Hill - Captain Illima - has a lvl 15 Gumshoos (evolves at lvl 20)
- Diglett's Tunnel - Black Belt Greg - has a lvl 23 Hariyama (evolves at lvl 24)
- Malie City (outer cape) - Janitor Shawn - has a lvl 27 Muk
- Route 12 - Scientist Jayson - has a lvl 29 Magneton
- Secluded Shore - Swimmer Robert - has a lvl 29 Whiscash (evolves at lvl 30)
- Route 16 - Scientist Reid - has a lvl 33 Muk
- Aether Paradise - Employee in Hallway - has a lvl 37 Muk
Ultra Sun & Ultra Moon:
- Almost all Totems
- Tapu Village - Veteran Angus - has a lvl 36 Sliggoo (evolves at lvl 40)
- Route 16 - Scientist Reid - has a lvl 33 Muk
- Poni Breaker Coast - Swimmer Derek has a lvl 44 Dragalge (evolves at lvl 44)
Sword & Shield:
- Motostoke Stadium - Kabu - has a lvl 27 Centiskorch (evolves at lvl 28)
- Stow on Side Stadium - (Shield) Allister - has a lvl 35 Cursola (evolves at lvl 38)
- Route 10 - Interviewers Gillian and Cam - have a lvl 46 Klinklang (evolves at lvl 49)
(I may have missed some, since I didn't use Bulbapedia source for SM/USUM/SWSH)
Game Move Errors
- RB - Lance - Dragonite - Barrier
Lance's Dragonite knows Barrier in Red & Blue, Dragonite couldn't know this move until Gen-6.
- GSC - Lance - Aerodactyl - Rock Slide
Lance's Aerodactyl knows Rock Slide in the Gen-2 games, Aerodactyl couldn't know this move until Gen-3.
- Emerald - Cooltrainer Dianne - Lanturn - Earthquake
In Pokémon Emerald's Victory Road, Cooltrainer Dianne's Lanturn knows Earthquake, Lanturn can't know this move.
- BW - School Kid Ann - Wormadam - Leaf Storm
On Unova Route 12, School Kid Ann's Sandy Cloak Wormadam knows the move Leaf Storm, Sandy Cloak cannot know this move, only Plant Cloak can.
- SM - Totem Wishiwashi - Ally Alomomola - Water Gun
In the battle against Totem Wishiwashi at Brooklet Hill, the Totem Pokémon can summon an ally Alomomola which knows Water Gun, Alomomola cannot know this move.
- LPGE - Hiker Jim - Onix - Wrap
In the Underground Path (Kanto Routes 7-8), Onix Master Jim has an Onix that knows Wrap, Onix cannot know this move.
Moves that are learnt at early level:
(In this list, we will look on to those moves learnt at early level, and the previous evolution/s can't learn this move at all, so it cannot be obtained by Breeding, thus it is illegal.)
- GSC - Pryce - Piloswine - Fury Attack (level 31, it should learn at level 33).
- RSE - Sidney - Shiftry - Extrasensory (level 48, although Shiftry doesn't learn this move until level 49 as a Nuzleaf).
- RSE - Winona - Altaria - Dragon Dance (level 33, it should learn at lvl 40).
- Emerald - Norman - Linoone - Slash (level 29, it should learn at lvl 41).
- Platinum - Bertha - Rhyperior - Rock Wrecker (level 55, it should learn at lvl 61).
- B2W2 - Bianca - Purrloin - Growl (level 2, it should learn at lvl 3).
- SwSh - Corvin - Dracozolt - Bolt Beak (level 58, it should learn at level 63)
- SwSh - Terry - Dracovish - Fishious Rend (Fishious Rend at level 57, it should learn at level 63).
Not included Battle Frontier since it is Post-Game.
Bulbapedia -> Game move errors
