Gold/Silver:

The code to fight any pokemon is 01XX23da. Replace the XX with the Pokémon you want with the codes below.

Crystal:

01: Bulbasaur
02: Ivysaur
03: Venusaur
04: Charmander
05: Charmeleon
06: Charizard
07: Squirtle
08: Wartortle
09: Blastoise
0A: Caterpie
0B: Metapod
0C: Butterfree
0D: Weedle
0E: Kakuna
0F: Beedrill
10: Pidgey
11: Pidgeotto
12: Pidgeot
13: Rattata
14: Raticate
15: Spearow
16: Fearow
17: Ekans
18: Arbok
19: Pikachu
1A: Raichu
1B: Sandshrew
1C: Sandslash
1D: Nidoran(F)
1E: Nidorina
1F: Nidoqueen
20: Nidoran(M)
21: Nidorino
22: Nidoking
23: Clefairy
24: Clefable
25: Vulpix
26: Ninetails
27: Jigglypuff
28: Wigglytuff
29: Zubat
2A: Golbat
2B: Oddish
2C: Gloom
2D: Vileplume
2E: Paras
2F: Parasect
30: Venonat
31: Venomoth
32: Diglett
33: Dugtrio
34: Meowth
35: Persian
36: Psyduck
37: Golduck
38: Mankey
39: Primeape
3a: Growlithe
3b: Arcanine
3c: Poliwag
3d: Poliwhirl
3e: Poliwrath
3f: Abra
40: Kadabra
41: Alakazam
42: Machop
43: Machoke
44: Machamp
45: Bellsprout
46: Weepinbell
47: Victreebell
48: Tentacool
49: Tentacruel
4A: Geodude
4B: Graveler
4C: Golem
4D: Ponyta
4E: Rapidash
4F: Slowpoke
50: Slowbro
51: Magnemite
52: Magneton
53: Farfetch'd
54: Doduo
55: Dodrio
56: Seel
57: Dewgong
58: Grimer
59: Muk
5A: Shellder
5B: Cloyster
5C: Gastly
5D: Haunter
5E: Gengar
5F: Onix
60: Drowzee
61: Hypno
62: Krabby
63: Kingler
64: Voltorb
65: Electrode
66: Exeggcute
67: Exeggcutor
68: Cubone
69: Marowak
6A: Hitmonlee
6B: Hitmonchan
6C: Lickitung
6D: Koffing
6E: Weezing
6F: Rhyhorn
70: Rhydon
71: Chansey
72: Tangela
73: Kangaskhan
74: Horsea
75: Seadra
76: Goldeen
77: Seaking
78: Staryu
79: Starmie
7A: Mr. Mime
7B: Scyther
7C: Jynx
7D: Electabuzz
7E: Magmar
7F: Pinsir
80: Tauros
81: Magikarp
82: Gyarados
83: Lapras
84: Ditto
85: Eevee
86: Vaporeon
87: Jolteon
88: Flareon
89: Porygon
8A: Omanyte
8B: Omastar
8C: Kabuto
8D: Kabutops
8E: Aerodactyl
8F: Snorlax
90: Articunno
91: Zapdos
92: Moltres
93: Dratini
94: Dragonair
95: Dragonite
96: Mewtwo
97: Mew
98: Chikorita
99: Bayleaf
9A: Meganium
9B: Cyndaquil
9C: Quilava
9D: Typhlosion
9E: Totodile
9F: Croconaw
A0: Feraligatr
A1: Sentret
A2: Furret
A3: Hoothoot
A4: Noctowl
A5: Ledyba
A6: Ledian
A7: Spinarak
A8: Ariados
A9: Crobat
Aa: Chinchou
Ab: Lanturn
Ac: Pichu
Ad: Igglybuff
Ae: Igglybuff
Af: Togepi
B0: Togetic
B1: Natu
B2: Xatu
B3: Mereep
B4: Flaaffy
B5: Ampharos
B6: Bellossom
B7: Marill
B8: Mariruri
B9: Sudowoodo
BA: Politoed
BB: Hoppip
BC: Skiploom
BD: Jumpluff
BE: Aipom
BF: Sunkern
C0: Sunflora
C1: Yanma
C2: Wooper
C3: Quagsire
C4: Espeon
C5: Umbreon
C6: Murkrow
C7: Slowking
C8: Misdreavus
C9: Unown
CA: Wobbuffet
CB: Girafarig
CC: Pineco
CD: Forretress
CE: Dunsparce
CF: Gligar
D0: Steelix
D1: Snubble
D2: Granbull
D3: Qwillfish
D4: Scizor
D5: Shuckle
D6: Heracross
D7: Sneasel
D8: Teddiursa
D9: Ursaring
DA: Slugma
DB: Magcargo
DC: Swinub
DD: Piloswine
DE: Corsola
DF: Remoraid
E0: Octillery
E1: Delibird
E2: Mantine
E3: Skarmory
E4: Houndour
E5: Houndoom
E6: Kingdra
E7: Phanpy
E8: Donphan
E9: Porygon2
EA: Stantler
EB: Smeargle
EC: Tyrogue
ED: Hitmontop
EE: Smoochum
EF: Elekid
F0: Magby
F1: Miltank
F2: Blissey
F3: Raikou
F4: Entei
F5: Suicune
F6: Larvitar
F7: Pupitar
F8: Tyranitar
F9: Lugia
FA: Ho-Oh
FB: Celebi