The Sinnoh Pokedex from Diamond and Pearl includes 493 Pokemon. Gen 4's biggest change is the physical/special split by move instead of type. Gyarados can finally use its 125 Attack on Water moves. The flipside: Sinnoh has a brutal Fire-type shortage. Infernape and Ponyta are basically it before postgame. Filter by type to see just how limited some coverage is.
Venomoth
Diglett
Dugtrio
Meowth
Persian
Psyduck
Golduck
Mankey
Primeape
Growlithe
Arcanine
Poliwag
Poliwhirl
Poliwrath
Abra
Kadabra
Alakazam
Machop
Machoke
Machamp
Bellsprout
Weepinbell
Victreebel
Tentacool
Tentacruel
Geodude
Graveler
Golem
Ponyta
Rapidash
Slowpoke
Slowbro
Magnemite
Magneton
Farfetch'd
Doduo
Dodrio
Seel
Dewgong
Grimer
Muk
Shellder
Cloyster
Gastly
Haunter
Gengar
Onix
Drowzee
493 total across Generations 1 through 4. The Sinnoh regional dex has 151 Pokemon (a nod to Gen 1). Many popular species are locked behind the National Dex, which you unlock after seeing all 150 Sinnoh Pokemon. The regional dex is notoriously limited, especially for Fire types.
Before Gen 4, whether a move was physical or special depended on the move's type. All Water moves were special, all Fighting moves were physical, regardless of the actual move. Diamond and Pearl changed it so each move has its own physical or special category. This lets Pokemon like Gyarados (125 Attack) finally use physical Water moves.
Chimchar. Infernape is Fire/Fighting with great Speed and mixed offenses. The reason it's the top pick: Sinnoh has almost no Fire types. Chimchar and Ponyta are your only options before postgame. Piplup evolves into Empoleon (Water/Steel, unique typing), and Turtwig's Torterra (Grass/Ground) is solid but slow.
The Sinnoh regional dex only includes two Fire-type evolutionary lines before the National Dex: Chimchar and Ponyta. If you didn't pick Chimchar, Ponyta is your only Fire-type option until postgame. Platinum fixed this by expanding the regional dex, but DP is stuck with the drought.
Generation 4, released in 2006 in Japan and 2007 in North America for the Nintendo DS. Introduced the physical/special split per move, online trading via the GTS, the Sinnoh region, and 107 new Pokemon including new evolutions for older species.
- Gen IY
Yellow151 Pokemon - Gen IRB

Red & Blue151 Pokemon - Gen IIGS

Gold & Silver251 Pokemon - Gen IIC
Crystal251 Pokemon - Gen IIIRS

Ruby & Sapphire386 Pokemon - Gen IIIFRLG

FireRed & LeafGreen386 Pokemon - Gen IIIE
Emerald386 Pokemon - Gen IVPt
Platinum493 Pokemon - Gen IVHGSS

HeartGold & SoulSilver493 Pokemon - Gen IVDP

Diamond & Pearl493 Pokemon - Gen VBW

Black & White649 Pokemon - Gen VB2W2

Black 2 & White 2649 Pokemon - Gen VIXY

X & Y721 Pokemon - Gen VIORAS

Omega Ruby & Alpha Sapphire721 Pokemon - Gen VIIUSUM

Ultra Sun & Ultra Moon807 Pokemon - Gen VIISM

Sun & Moon802 Pokemon - Gen VIILGPE

Let's Go Pikachu & Eevee153 Pokemon - Gen VIIISwSh

Sword & Shield664 Pokemon - Gen VIIIPLA
Legends: Arceus242 Pokemon - Gen VIIIBDSP

Brilliant Diamond & Shining Pearl493 Pokemon - Gen IXSV

Scarlet & Violet733 Pokemon - Gen IXLZA
Legends: Z-A364 Pokemon - Gen XWW

Winds & Waves51 Pokemon