In Pokémon Gold & Silver, Genders were created to allow for breeding of the Pokémon. Genders have since become a staple of the Pokémon World. There were no differences between Genders in the games, except for the Nidoran line...until now.
Diamond & Pearl have brought the feature of Gender differences to the table. However, this is not utilised for every Pokémon, and not all are visible from the front. Only one fifth of the Pokémon have differences between gender. In the game, if you have seen the differences, you can check in part of the Pokédex as seen in the picture here. However, below is a list of all of the Pokémon with differences and a note as to what the difference is

National No. Male Pic Female Pic US Name Difference Description
#003 Venusaur Female Venusaur's Flower has a Seed in it
#012 Butterfree Butterfree's Wing Pattern is different
#019 Rattata Female Rattata's Whiskers are Smaller
#020 Raticate Female Raticate's Whiskers are Smaller
#025 Pikachu Female Pikachu's Tail has a groove in it
#026 Raichu Female Raichu's Tail lacks the point at the end
#041 Zubat Female Zubat's teeth are smaller
#042 Golbat Female Golbat's teeth are smaller
#044 Gloom Female Gloom has a different flower pattern
#045 Vileplume Female Vileplume has a different flower pattern
#064 Kadabra Female Kadabra has a smaller moustache
#065 Alakazam Female Alakazam has a smaller moustache
#084 Doduo Female Doduo have beige necks. Male Doduo have black necks
#085 Dodrio Female Dodrio have beige necks. Male Dodrio have black necks
#097 Hypno Male Hypno has a smaller collar
#111 Rhyhorn Female Rhyhorn has a smaller horn
#112 Rhydon Female Rhydon has a smaller horn
#118 Goldeen Female Goldeen has a smaller horn
#119 Seaking Female Seaking has a smaller horn
#123 Scyther Female Scyther has a bigger abdomen
#129 Magikarp Female Magikarp has white whiskers
#130 Gyarados Female Gyarados has white whiskers
#154 Meganium Female Meganium has smaller antennae
#165 Ledyba Female Ledyba has smaller antennae
#166 Ledian Female Ledian has smaller antennae
#178 Xatu Female Xatu has less stripes on her body
#185 Sudowoodo Female Sudowoodo has smaller antennae
#186 Politoed Female Politoed has smaller spots on their cheek
#190 Aipom Female Aipom has longer hair
#194 Wooper Female Wooper have less bars on their antennae
#195 Quagsire Female Quagsire have smaller back-fins
#198 Murkrow Female Murkrow has a smaller 'hat'
#202 Wobbuffet Female Wobbufet appear to wear lipstick
#203 Girafarig Female Girafarig have a bigger yellow pattern on their body
#207 Gligar Male Gligar have a bigger stinger
#208 Steelix Male Steelix have extra teeth
#112 Scizor Female Scizor have a bigger abdomen
#214 Heracross Female Heracross' horns are shaped similar to hearts
#215 Sneasel Female Sneasel have smaller head feathers
#217 Ursaring Male Ursaring's have longer shoulder hair
#221 Piloswine Female Piloswine have smaller tusks
#224 Octillery Female Octiller have smaller yellow bumps & feelers
#229 Houndoom Female Houndoom have smaller horns
#232 Donphan Female Donphan have smaller tusks
#256 Combusken Female Combusken have smaller head feathers
#257 Blaziken Female Blaziken have smaller head feathers
#267 Beautifly The red spot on Female Beautifly is smaller
#269 Dustox Female Dustox have smaller antennae
#272 Ludicolo Female Ludicolo's Body Stripes are thinner
#274 Nuzleaf Female Nuzleaf have smaller head leaves
#275 Shiftry Female Shiftry have smaller hand-leaves
#307 Meditite Female Meditite's ears are lower on their head
#308 Medicham The top of Female Medicham's head is smaller
#315 Roselia Female Roselia have bigger body leaves
#316 Gulpin Female Gulpin have smaller head feathers
#317 Swalot Female Swalot have smaller whiskers
#322 Numel Female Numel have bigger humps
#323 Camerupt Female Camerupt have bigger Volcanic Humps
#332 Cacturne Female Cacturne has a giant diamond on their body
#350 Milotic Female Miltoic have larger eyebrows
#369 Relicanth Female Relicanth have smaller jaws
#396 Starly Male Starly have larger foreheads
#397 Staravia Male Staravia have larger foreheads
#398 Staraptor Male Staraptor have larger foreheads
#399 Bidoof Male Bidoof have extra lumps to their tails
#400 Bibarel Male Bibarel have bigger faces
#401 Kricketot Female Kricketot have larger collars
#402 Kricketune Female Kricketune have smaller moustaches
#403 Shinx Female Shinx have shorter head hair
#404 Luxio Female Luxio have shorter head hair
#405 Luxray Female Luxray have shorter head hair
#407 Roserade Female Roserade have larger capes
#415 Combee Female Combee have a Red spot on the bottom face's forehead
#417 Pachirisu Male Pachirisu's forehead pattern is longer
#418 Buizel Male Buizel have two stripes on their back. Female Buizel only have one.
#419 Floatzel Male Floatzel have two stripes on their back. Female Floatzel only have one.
#424 Ambipom Female Ambipom have longer head hair
#443 Gible Male Gible have a groove in their fin
#444 Gabite Male Gabite have a groove in their fin
#445 Garchomp Male Garchomp have a groove in their fin
#449 Hippopotas Male & Female Hippoptas have their colour scheme switched
#450 Hippowdon Male Hippowdon are brown. Female Hippowdon are Grey
#453 Croagunk Female Croagunk's have smaller chests
#454 Toxicroak Male Toxicroak have larger chins
#456 Finneon Female Finneon have larger tails
#457 Lumineon Female Lumineon have larger fins
#459 Snover Female Snover's trunks are more snow covered
#460 Abomasnow Female Abomasnow have larger breast hair
#461 Weavile Male Weavile have larger ear feathers
#464 Rhyperior Female Rhyperior have smaller horns
#465 Tangrowth Female Tangrowth have larger hands
#473 Mamoswine Male Mamoswine have larger tusks
 
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